Error while using Intent Extras : <p>I am using an app where initially , there is a intro screen which runs only when the app is started for the first time. At the last slide , I have two buttons namely signInAsCustomer and signInAsSeller . Both the intents take you to the same activity , but passes two different intent extras . signInAsCustomer passes extra with key "type" and value "customer" while signInAsSeler passes an extra with key "type" and value "seller" . When the second activity is started  , I get the extra and depending on the extra , I set some text in a specific text view. The code for the SignInActivity is below.</p>

<p><strong>SignInActivity</strong></p>

<pre><code>private String type;
private TextView tv_seller_or_customer , forgot_password , guest_login ;
private EditText email , password ;
private Button login , signUp;
private FirebaseAuth mAuth;
private FirebaseAuth.AuthStateListener mAuthStateListener;

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

    // Initialising Firebase Auth Object
    mAuth = FirebaseAuth.getInstance();

    // This is the Auth state listener .  It checks if the user is aldready signed In or not
    mAuthStateListener = new FirebaseAuth.AuthStateListener() {
        @Override
        public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
            FirebaseUser user = firebaseAuth.getCurrentUser();
            if (user != null)
            {
                // User is signed in
                // Goes to a new activity
                Intent i = new Intent(SignInActivity.this , MainActivity.class);
                startActivity(i);
            }else
            {
                // User is signed out
                // Shows sign in page to the user
                signInUser();
            }
        }
    };


}

@Override
protected void onResume() {
    super.onResume();
    mAuth.addAuthStateListener(mAuthStateListener);
}

@Override
protected void onPause() {
    super.onPause();
    if (mAuthStateListener != null)
    {
        mAuth.removeAuthStateListener(mAuthStateListener);
    }
}

public void signInUser()
{
    // This method initialises all the views in the signInPage
    initialiseViews();
    // Getting the intent extras and changing the activity depending upon the button pressed in the Intro Slider
   Intent i = getIntent();
    if (i != null) {
        type = i.getStringExtra("type");
        if (type.equals("seller")) {
            tv_seller_or_customer.setText("Seller");
            initialiseViews();
            guest_login.setVisibility(View.GONE);
        } else {
            tv_seller_or_customer.setText("Customer");
            initialiseViews();
        }
    }else
    {
        tv_seller_or_customer.setVisibility(View.GONE);
    }

    // Listener for loging in 
    login.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            String emailString = email.getText().toString();
            String passwordString = password.getText().toString().trim();
            if (!Is_Valid_Email(emailString))
            {
                Toast.makeText(SignInActivity.this , "Please enter a valid Email ID", Toast.LENGTH_SHORT).show();
            }else {
                loginUser(emailString , passwordString);
            }
        }
    });

}

public boolean Is_Valid_Email(String email_check) {
    return !email_check.isEmpty() &amp;&amp; isEmailValid(email_check);
}

boolean isEmailValid(CharSequence email) {
    return android.util.Patterns.EMAIL_ADDRESS.matcher(email)
            .matches();
}

public void loginUser(String emailLogin , String passwordLogin)
{
    mAuth.signInWithEmailAndPassword(emailLogin , passwordLogin)
            .addOnCompleteListener(this, new OnCompleteListener&lt;AuthResult&gt;() {
                @Override
                public void onComplete(@NonNull Task&lt;AuthResult&gt; task) {
                    if (!task.isSuccessful())
                    {
                        Log.w("sign in", "Failed" , task.getException());
                    }else
                    {
                        Intent i = new Intent(SignInActivity.this , MainActivity.class);
                        startActivity(i);
                    }

                }
            });

}

public void initialiseViews()
{
   forgot_password = (TextView) findViewById(R.id.forgotPassword);
    guest_login = (TextView) findViewById(R.id.guest_tv);
    email = (EditText) findViewById(R.id.email_edt);
    password = (EditText) findViewById(R.id.pass_edt);
    login = (Button) findViewById(R.id.login_button);
    signUp = (Button) findViewById(R.id.sign_up_btn);
}}
</code></pre>

<p>The error I'm getting </p>

<pre><code>    E/AndroidRuntime: FATAL EXCEPTION: main
              Process: com.example.ankit_pc.pharmahouse, PID: 15750
              java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
                  at com.example.ankit_pc.pharmahouse.SignInActivity.signInUser(SignInActivity.java:78)
                  at com.example.ankit_pc.pharmahouse.SignInActivity$1.onAuthStateChanged(SignInActivity.java:48)
                  at com.google.firebase.auth.FirebaseAuth$1.run(Unknown Source)
                  at android.os.Handler.handleCallback(Handler.java:739)
                  at android.os.Handler.dispatchMessage(Handler.java:95)
                  at android.os.Looper.loop(Looper.java:148)
                  at android.app.ActivityThread.main(ActivityThread.java:7325)
                  at java.lang.reflect.Method.invoke(Native Method)
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120) Application terminated.
</code></pre>

<p>Where have I gone wrong?</p>
