Android Error [java.lang.NullPointerException: 'void android.support.v7.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference] : <p>I have my main which contain a menu drawer with some fragments. Then a host page before to display the main. I have some errors about that <code>Attempt to invoke virtual method 'void android.support.v7.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference</code></p>

<p>I tried to change it <code>getSupportActionBar().setDisplayHomeAsUpEnabled(true);</code>
but still nothing even with <code>public class MainActivity extends Activity {</code> or <code>public class MainActivity extends AppcompactActivity {</code></p>

<p>There is my main:</p>

<pre><code>    package thyroid.com.thyroid;


import thyroid.com.thyroid.adapter.NavDrawerListAdapter;
import thyroid.com.thyroid.model.NavDrawerItem;

import java.util.ArrayList;

import android.support.v7.app.ActionBarActivity;
import android.app.Activity;
import android.app.Fragment;
import android.app.FragmentManager;
import android.content.res.Configuration;
import android.content.res.TypedArray;
import android.net.Uri;
import android.os.Bundle;
import android.support.v4.app.ActionBarDrawerToggle;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBar;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ListView;

import com.google.android.gms.appindexing.Action;
import com.google.android.gms.appindexing.AppIndex;
import com.google.android.gms.common.api.GoogleApiClient;


public class MainActivity extends Activity {
    private DrawerLayout mDrawerLayout;
    private ListView mDrawerList;
    private ActionBarDrawerToggle mDrawerToggle;

    // nav drawer title
    private CharSequence mDrawerTitle;

    // used to store app title
    private CharSequence mTitle;

    // slide menu items
    private String[] navMenuTitles;
    private TypedArray navMenuIcons;

    private ArrayList&lt;NavDrawerItem&gt; navDrawerItems;
    private NavDrawerListAdapter adapter;
    /**
     * ATTENTION: This was auto-generated to implement the App Indexing API.
     * See https://g.co/AppIndexing/AndroidStudio for more information.
     */
    private GoogleApiClient client;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mTitle = mDrawerTitle = getTitle();

        // load slide menu items
        navMenuTitles = getResources().getStringArray(R.array.nav_drawer_items);

        // nav drawer icons from resources
        navMenuIcons = getResources()
                .obtainTypedArray(R.array.nav_drawer_icons);

        mDrawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);
        mDrawerList = (ListView) findViewById(R.id.list_slidermenu);

        navDrawerItems = new ArrayList&lt;NavDrawerItem&gt;();

        // adding nav drawer items to array
        // Home
        navDrawerItems.add(new NavDrawerItem(navMenuTitles[0], navMenuIcons.getResourceId(0, -1)));
        // Find People
        navDrawerItems.add(new NavDrawerItem(navMenuTitles[1], navMenuIcons.getResourceId(1, -1)));
        // Photos
        navDrawerItems.add(new NavDrawerItem(navMenuTitles[2], navMenuIcons.getResourceId(2, -1)));
        // Communities, Will add a counter here
        navDrawerItems.add(new NavDrawerItem(navMenuTitles[3], navMenuIcons.getResourceId(3, -1), true, "22"));
        // Pages
        navDrawerItems.add(new NavDrawerItem(navMenuTitles[4], navMenuIcons.getResourceId(4, -1)));
        // What's hot, We  will add a counter here
        navDrawerItems.add(new NavDrawerItem(navMenuTitles[5], navMenuIcons.getResourceId(5, -1), true, "50+"));


        // Recycle the typed array
        navMenuIcons.recycle();

        mDrawerList.setOnItemClickListener(new SlideMenuClickListener());

        // setting the nav drawer list adapter
        adapter = new NavDrawerListAdapter(getApplicationContext(),
                navDrawerItems);
        mDrawerList.setAdapter(adapter);

        // enabling action bar app icon and behaving it as toggle button
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
        getSupportActionBar().setHomeButtonEnabled(true);

        mDrawerToggle = new ActionBarDrawerToggle(this, mDrawerLayout,
                R.drawable.ic_drawer, //nav menu toggle icon
                R.string.app_name, // nav drawer open - description for accessibility
                R.string.app_name // nav drawer close - description for accessibility
        ) {
            public void onDrawerClosed(View view) {
                getSupportActionBar().setTitle(mTitle);
                // calling onPrepareOptionsMenu() to show action bar icons
                invalidateOptionsMenu();
            }

            public void onDrawerOpened(View drawerView) {
                getSupportActionBar().setTitle(mDrawerTitle);
                // calling onPrepareOptionsMenu() to hide action bar icons
                invalidateOptionsMenu();
            }
        };
        mDrawerLayout.setDrawerListener(mDrawerToggle);

        if (savedInstanceState == null) {
            // on first time display view for first nav item
            displayView(0);
        }
        // ATTENTION: This was auto-generated to implement the App Indexing API.
        // See https://g.co/AppIndexing/AndroidStudio for more information.
        client = new GoogleApiClient.Builder(this).addApi(AppIndex.API).build();
    }

    private ActionBar getSupportActionBar() {
        return null;
    }

    @Override
    public void onStart() {
        super.onStart();

        // ATTENTION: This was auto-generated to implement the App Indexing API.
        // See https://g.co/AppIndexing/AndroidStudio for more information.
        client.connect();
        Action viewAction = Action.newAction(
                Action.TYPE_VIEW, // TODO: choose an action type.
                "Main Page", // TODO: Define a title for the content shown.
                // TODO: If you have web page content that matches this app activity's content,
                // make sure this auto-generated web page URL is correct.
                // Otherwise, set the URL to null.
                Uri.parse("http://host/path"),
                // TODO: Make sure this auto-generated app URL is correct.
                Uri.parse("android-app://thyroid.com.thyroid/http/host/path")
        );
        AppIndex.AppIndexApi.start(client, viewAction);
    }

    @Override
    public void onStop() {
        super.onStop();

        // ATTENTION: This was auto-generated to implement the App Indexing API.
        // See https://g.co/AppIndexing/AndroidStudio for more information.
        Action viewAction = Action.newAction(
                Action.TYPE_VIEW, // TODO: choose an action type.
                "Main Page", // TODO: Define a title for the content shown.
                // TODO: If you have web page content that matches this app activity's content,
                // make sure this auto-generated web page URL is correct.
                // Otherwise, set the URL to null.
                Uri.parse("http://host/path"),
                // TODO: Make sure this auto-generated app URL is correct.
                Uri.parse("android-app://thyroid.com.thyroid/http/host/path")
        );
        AppIndex.AppIndexApi.end(client, viewAction);
        client.disconnect();
    }

    /**
     * Slide menu item click listener
     */
    private class SlideMenuClickListener implements
            ListView.OnItemClickListener {
        @Override
        public void onItemClick(AdapterView&lt;?&gt; parent, View view, int position,
                                long id) {
            // display view for selected nav drawer item
            displayView(position);
        }
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // toggle nav drawer on selecting action bar app icon/title
        if (mDrawerToggle.onOptionsItemSelected(item)) {
            return true;
        }
        // Handle action bar actions click
        switch (item.getItemId()) {
            case R.id.action_settings:
                return true;
            default:
                return super.onOptionsItemSelected(item);
        }
    }

    /* *
     * Called when invalidateOptionsMenu() is triggered
     */
    @Override
    public boolean onPrepareOptionsMenu(Menu menu) {
        // if nav drawer is opened, hide the action items
        boolean drawerOpen = mDrawerLayout.isDrawerOpen(mDrawerList);
        menu.findItem(R.id.action_settings).setVisible(!drawerOpen);
        return super.onPrepareOptionsMenu(menu);
    }

    /**
     * Diplaying fragment view for selected nav drawer list item
     */
    private void displayView(int position) {
        // update the main content by replacing fragments
        Fragment fragment = null;
        switch (position) {
            case 0:
                fragment = new LoginFragment();
                break;
            case 1:
                fragment = new RegisterFragment();
                break;
            case 2:
                fragment = new LoginFragment();
                break;
            case 3:
                fragment = new LoginFragment();
                break;
            case 4:
                fragment = new LoginFragment();
                break;
            case 5:
                fragment = new LoginFragment();
                break;

            default:
                break;
        }

        if (fragment != null) {
            FragmentManager fragmentManager = getFragmentManager();
            fragmentManager.beginTransaction()
                    .replace(R.id.frame_container, fragment).commit();

            // update selected item and title, then close the drawer
            mDrawerList.setItemChecked(position, true);
            mDrawerList.setSelection(position);
            setTitle(navMenuTitles[position]);
            mDrawerLayout.closeDrawer(mDrawerList);
        } else {
            // error in creating fragment
            Log.e("MainActivity", "Error in creating fragment");
        }
    }

    @Override
    public void setTitle(CharSequence title) {
        mTitle = title;
        getSupportActionBar().setTitle(mTitle);
    }

    /**
     * When using the ActionBarDrawerToggle, you must call it during
     * onPostCreate() and onConfigurationChanged()...
     */

    @Override
    protected void onPostCreate(Bundle savedInstanceState) {
        super.onPostCreate(savedInstanceState);
        // Sync the toggle state after onRestoreInstanceState has occurred.
        mDrawerToggle.syncState();
    }

    @Override
    public void onConfigurationChanged(Configuration newConfig) {
        super.onConfigurationChanged(newConfig);
        // Pass any configuration change to the drawer toggls
        mDrawerToggle.onConfigurationChanged(newConfig);
    }

}
</code></pre>

<p>Here one of the fragment:</p>

<pre><code>    package thyroid.com.thyroid;


import android.app.Fragment;
import android.app.FragmentTransaction;
import android.content.SharedPreferences;
import android.os.Bundle;
import android.support.design.widget.Snackbar;
import android.support.v7.widget.AppCompatButton;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.EditText;
import android.widget.ProgressBar;
import android.widget.TextView;

import retrofit2.Call;
import retrofit2.Callback;
import retrofit2.Retrofit;
import retrofit2.converter.gson.GsonConverterFactory;
import thyroid.com.thyroid.model.ServerRequest;
import thyroid.com.thyroid.model.ServerResponse;
import thyroid.com.thyroid.model.User;


public class LoginFragment extends Fragment implements View.OnClickListener{

    private AppCompatButton btn_login;
    private EditText et_email,et_password;
    private TextView tv_register,tv_reset_password;
    private ProgressBar progress;
    private SharedPreferences pref;


    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

        View view = inflater.inflate(R.layout.fragment_login,container,false);
        initViews(view);
        return view;
    }

    private void initViews(View view){

        pref = getActivity().getPreferences(0);

        btn_login = (AppCompatButton)view.findViewById(R.id.btn_login);
        tv_register = (TextView)view.findViewById(R.id.tv_register);
        tv_reset_password = (TextView)view.findViewById(R.id.tv_reset_password);
        et_email = (EditText)view.findViewById(R.id.et_email);
        et_password = (EditText)view.findViewById(R.id.et_password);

        progress = (ProgressBar)view.findViewById(R.id.progress);

        btn_login.setOnClickListener(this);
        tv_register.setOnClickListener(this);
        tv_reset_password.setOnClickListener(this);
    }

    @Override
    public void onClick(View v) {

        switch (v.getId()){

            case R.id.tv_register:
                goToRegister();
                break;

            case R.id.btn_login:
                String email = et_email.getText().toString();
                String password = et_password.getText().toString();

                if(!email.isEmpty() &amp;&amp; !password.isEmpty()) {

                    progress.setVisibility(View.VISIBLE);
                    loginProcess(email,password);

                } else {

                    Snackbar.make(getView(), "Fields are empty !", Snackbar.LENGTH_LONG).show();
                }
                break;
            case R.id.tv_reset_password:
                goToResetPassword();
                break;
        }
    }
    private void loginProcess(String email,String password){

        Retrofit retrofit = new Retrofit.Builder()
                .baseUrl(Constants.BASE_URL)
                .addConverterFactory(GsonConverterFactory.create())
                .build();

        RequestInterface requestInterface = retrofit.create(RequestInterface.class);

        User user = new User();
        user.setEmail(email);
        user.setPassword(password);
        ServerRequest request = new ServerRequest();
        request.setOperation(Constants.LOGIN_OPERATION);
        request.setUser(user);
        Call&lt;ServerResponse&gt; response = requestInterface.operation(request);

        response.enqueue(new Callback&lt;ServerResponse&gt;() {
            @Override
            public void onResponse(Call&lt;ServerResponse&gt; call, retrofit2.Response&lt;ServerResponse&gt; response) {

                ServerResponse resp = response.body();
                Snackbar.make(getView(), resp.getMessage(), Snackbar.LENGTH_LONG).show();

                if(resp.getResult().equals(Constants.SUCCESS)){
                    SharedPreferences.Editor editor = pref.edit();
                    editor.putBoolean(Constants.IS_LOGGED_IN,true);
                    editor.putString(Constants.EMAIL,resp.getUser().getEmail());
                    editor.putString(Constants.NAME,resp.getUser().getName());
                    editor.putString(Constants.UNIQUE_ID,resp.getUser().getUnique_id());
                    editor.apply();
                    goToProfile();

                }
                progress.setVisibility(View.INVISIBLE);
            }

            @Override
            public void onFailure(Call&lt;ServerResponse&gt; call, Throwable t) {

                progress.setVisibility(View.INVISIBLE);
                Log.d(Constants.TAG,"failed");
                Snackbar.make(getView(), t.getLocalizedMessage(), Snackbar.LENGTH_LONG).show();

            }
        });
    }

    private void goToResetPassword(){

        Fragment reset = new Fragment();
        FragmentTransaction ft = getFragmentManager().beginTransaction();
        ft.replace(R.id.fragment_frame,reset);
        ft.commit();
    }

    private void goToRegister(){

        Fragment register = new RegisterFragment();
        FragmentTransaction ft = getFragmentManager().beginTransaction();
        ft.replace(R.id.fragment_frame,register);
        ft.commit();
    }

    private void goToProfile(){

        Fragment profile = new ProfileFragment();
        FragmentTransaction ft = getFragmentManager().beginTransaction();
        ft.replace(R.id.fragment_frame,profile);
        ft.commit();
    }
}
</code></pre>

<p>And now the .class which is the "host page" before the main:</p>

<pre><code>package thyroid.com.thyroid;


import android.content.Intent;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;

import static thyroid.com.thyroid.R.id.button;


public class SplashScreen extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        super.onCreate(savedInstanceState);
        setContentView(R.layout.splashscreen);

        Button button = (Button) findViewById(R.id.button);
        assert button != null;
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent intent = new Intent(v.getContext(),MainActivity.class);
                startActivityForResult(intent,0);
            }
        });




        Thread timerThread = new Thread() {
            public void run() {
                try {
                    sleep(6000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } finally {
                    Intent intent = new Intent(SplashScreen.this, MainActivity.class);
                    startActivity(intent);
                }
            }
        };
        timerThread.start();
    }

    @Override
    protected void onPause() {
        // TODO Auto-generated method stub
        super.onPause();
        finish();
    }


}
</code></pre>

<p>This is my errors:</p>

<pre><code>06-20 11:23:33.603 29939-29939/thyroid.com.thyroid E/AndroidRuntime: FATAL EXCEPTION: main
                                                             Process: thyroid.com.thyroid, PID: 29939
                                                             java.lang.RuntimeException: Unable to start activity ComponentInfo{thyroid.com.thyroid/thyroid.com.thyroid.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference
                                                                 at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2298)
                                                                 at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
                                                                 at android.app.ActivityThread.access$800(ActivityThread.java:144)
                                                                 at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
                                                                 at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                 at android.os.Looper.loop(Looper.java:135)
                                                                 at android.app.ActivityThread.main(ActivityThread.java:5221)
                                                                 at java.lang.reflect.Method.invoke(Native Method)
                                                                 at java.lang.reflect.Method.invoke(Method.java:372)
                                                                 at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:898)
                                                                 at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:693)
                                                              Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference
                                                                 at thyroid.com.thyroid.MainActivity.onCreate(MainActivity.java:101)
                                                                 at android.app.Activity.performCreate(Activity.java:5976)
                                                                 at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
                                                                 at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2251)
                                                                 at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360) 
                                                                 at android.app.ActivityThread.access$800(ActivityThread.java:144) 
                                                                 at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278) 
                                                                 at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                                 at android.os.Looper.loop(Looper.java:135) 
                                                                 at android.app.ActivityThread.main(ActivityThread.java:5221) 
                                                                 at java.lang.reflect.Method.invoke(Native Method) 
                                                                 at java.lang.reflect.Method.invoke(Method.java:372) 
                                                                 at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:898) 
                                                                 at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:693) 
</code></pre>
