app crashing by pressing buttons in activity bar : Im new in android studio, the problem that i have is that when when i start the app and press a button in the activity bar it crashes

these is my main activity `

    package com.example.tirir_000.iavq;
    
    import android.net.Uri;
    import android.os.Bundle;
    import android.support.design.widget.FloatingActionButton;
    import android.support.design.widget.Snackbar;
    import android.support.v4.app.Fragment;
    import android.support.v4.app.FragmentTransaction;
    import android.app.FragmentManager;
    import android.view.View;
    import android.support.design.widget.NavigationView;
    import android.support.v4.view.GravityCompat;
    import android.support.v4.widget.DrawerLayout;
    import android.support.v7.app.ActionBarDrawerToggle;
    import android.support.v7.app.AppCompatActivity;
    import android.support.v7.widget.Toolbar;
    import android.view.Menu;
    import android.view.MenuItem;
    
    public class MainActivity extends AppCompatActivity
            implements NavigationView.OnNavigationItemSelectedListener, AdmisionFragment.OnFragmentInteractionListener {
            NavigationView navigationView = null;
            Toolbar toolbar = null;
    
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
            Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
            setSupportActionBar(toolbar);
    
            FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
            fab.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View view) {
                    Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                            .setAction("Action", null).show();
                }
            });
    
            DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
            ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
                    this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
            drawer.setDrawerListener(toggle);
            toggle.syncState();
    
            NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
            navigationView.setNavigationItemSelectedListener(this);
        }
    
        @Override
        public void onBackPressed() {
            DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
            if (drawer.isDrawerOpen(GravityCompat.START)) {
                drawer.closeDrawer(GravityCompat.START);
            } else {
                super.onBackPressed();
            }
        }
    
        @Override
        public boolean onCreateOptionsMenu(Menu menu) {
            // Inflate the menu; this adds items to the action bar if it is present.
            getMenuInflater().inflate(R.menu.main, menu);
            return true;
        }
    
        @Override
        public boolean onOptionsItemSelected(MenuItem item) {
            // Handle action bar item clicks here. The action bar will
            // automatically handle clicks on the Home/Up button, so long
            // as you specify a parent activity in AndroidManifest.xml.
            int id = item.getItemId();
    
            //noinspection SimplifiableIfStatement
            if (id == R.id.action_settings) {
                return true;
            }
    
            return super.onOptionsItemSelected(item);
        }
    
        @SuppressWarnings("StatementWithEmptyBody")
        @Override
        public boolean onNavigationItemSelected(MenuItem item) {
            // Handle navigation view item clicks here.
            int id = item.getItemId();
    
            if (id == R.id.nav_camera) {
                InicioFragment fragment = new InicioFragment();
                android.support.v4.app.FragmentTransaction fragmentTransaction =
                        getSupportFragmentManager().beginTransaction();
                fragmentTransaction.replace(R.id.fragment_container, fragment);
                fragmentTransaction.commit();
    
            } else if (id == R.id.nav_gallery) {
                AdmisionFragment fragment = new AdmisionFragment();
                android.support.v4.app.FragmentTransaction fragmentTransaction =
                        getSupportFragmentManager().beginTransaction();
                fragmentTransaction.replace(R.id.fragment_container, fragment);
                fragmentTransaction.commit();
    
            } else if (id == R.id.nav_slideshow) {
                CarrerasFragment fragment = new CarrerasFragment();
                android.support.v4.app.FragmentTransaction fragmentTransaction =
                        getSupportFragmentManager().beginTransaction();
                fragmentTransaction.replace(R.id.fragment_container, fragment);
                fragmentTransaction.commit();
    
            } else if (id == R.id.nav_manage) {
                ComunidadFragment fragment = new ComunidadFragment();
                android.support.v4.app.FragmentTransaction fragmentTransaction =
                        getSupportFragmentManager().beginTransaction();
                fragmentTransaction.replace(R.id.fragment_container, fragment);
                fragmentTransaction.commit();
    
            } else if (id == R.id.nav_share) {
    
            } else if (id == R.id.nav_send) {
                SgaFragment fragment = new SgaFragment();
                android.support.v4.app.FragmentTransaction fragmentTransaction =
                        getSupportFragmentManager().beginTransaction();
                fragmentTransaction.replace(R.id.fragment_container, fragment);
                fragmentTransaction.commit();
    
            }
    
            DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
            drawer.closeDrawer(GravityCompat.START);
            return true;
        }
    
        @Override
        public void onFragmentInteraction(Uri uri) {
    
        }

`}

please anyone help me, i am new at this and i cant fix it 