Getting NPE Unable to start activity: Attempt to invoke virtual method on a null object reference even with setContentView

Question

I'm following this tutorial to explore Firebase and use email authentication using three java classes

1. MainActivity
2. RegisterActivity
3. ProfileActivity

and three layout views.

1. activity_main.xml
2. activity_register.xml
3. profilepage.xml

[Youtube tutorial]1 I'm getting

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.firebaselearning/com.example.firebaselearning.RegisterActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference

full error

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.firebaselearning, PID: 8008
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.firebaselearning/com.example.firebaselearning.RegisterActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2913)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808)
    at android.os.Handler.dispatchMessage(Handler.java:106)
    at android.os.Looper.loop(Looper.java:193)
    at android.app.ActivityThread.main(ActivityThread.java:6669)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
    at com.example.firebaselearning.RegisterActivity.onCreate(RegisterActivity.java:60)
    at android.app.Activity.performCreate(Activity.java:7136)
    at android.app.Activity.performCreate(Activity.java:7127)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2893)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048) 
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78) 
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108) 
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808) 
    at android.os.Handler.dispatchMessage(Handler.java:106) 
    at android.os.Looper.loop(Looper.java:193) 
    at android.app.ActivityThread.main(ActivityThread.java:6669) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858) 

following are the codes (excluding gradle because i don't think it's where error originates from)

MainActivity.java

public class MainActivity extends AppCompatActivity {

//views
Button mRegisterBtn, mLogInBtn;

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

    //initialise view
    mRegisterBtn = findViewById(R.id.registerbutton);
    mLogInBtn = findViewById(R.id.login);

    //handle register button click
    mRegisterBtn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            //start register activity
            startActivity(new Intent(MainActivity.this, RegisterActivity.class));
        }
    });
  }
}

RegisterActivity.java

public class RegisterActivity extends AppCompatActivity {

//views
EditText mEmailET, mPassword;
Button mRegistrationBTN;

//progressbar to display progress which registering
ProgressDialog progressDialog;
//declare instance of fireAuth
private FirebaseAuth mAuth;

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

    //actionbar and its title
    ActionBar actionBar = getSupportActionBar();
    actionBar.setTitle("Create Account");

    //enable back button
    actionBar.setDisplayHomeAsUpEnabled(true);
    actionBar.setDisplayShowHomeEnabled(true);

    //init
    mEmailET = findViewById(R.id.editTextTextEmailAddress);
    mPassword = findViewById(R.id.editTextTextPassword);
    mRegistrationBTN = findViewById(R.id.registerbutton);

    //initialising firebaseAuth instance
    mAuth = FirebaseAuth.getInstance();

    progressDialog = new ProgressDialog(this);
    progressDialog.setMessage("Registering user ...");


    //handle register btn click
    mRegistrationBTN.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            //input email, password
            String email = mEmailET.getText().toString().trim();
            String password = mPassword.getText().toString().trim();
            //validate
            if(Patterns.EMAIL_ADDRESS.matcher(email).matches()){
                //show error and focus to edittext
                mEmailET.setError("Invalid Email");
                mEmailET.setFocusable(true);
            }else if (password.length()<6){
                //set error and focus to password
                mPassword.setError("Password length must be at least 6 characters");
                mPassword.setFocusable(true);
            }else{
                //register the user
                registerUser(email,password);
            }

        }
    });

}

private void registerUser(String email, String password){
    //email and password is valid ,shows progress dialog and start registering user
    progressDialog.show();

    mAuth.createUserWithEmailAndPassword(email,password)
            .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
                @Override
                public void onComplete(@NonNull Task<AuthResult> task) {
                    if (task.isSuccessful()) {
                        // Sign in success, dismiss dialog and start register activity
                        progressDialog.dismiss();
                        FirebaseUser user = mAuth.getCurrentUser();
                        assert user != null;
                        Toast.makeText(RegisterActivity.this,"Registered..."+user.getEmail(), Toast.LENGTH_SHORT).show();
                        startActivity(new Intent(RegisterActivity.this, ProfileActivity.class));
                        finish();
                    } else {
                        // If sign in fails, display a message to the user.
                        progressDialog.dismiss();
                        Toast.makeText(RegisterActivity.this, "Authentication failed.", Toast.LENGTH_SHORT).show();
                    }
                }
            }) .addOnFailureListener(new OnFailureListener() {
        @Override
        public void onFailure(@NonNull Exception e) {
            //error, dismiss progrss dialog and get and show error message
            progressDialog.dismiss();
            Toast.makeText(RegisterActivity.this,""+e.getMessage(),Toast.LENGTH_SHORT).show();
        }
    });
}
@Override
public boolean onSupportNavigateUp() {
    onBackPressed(); //go previous activity on clicking back
    return super.onSupportNavigateUp();
  }
}

I've reverified that names of my xml files are same .

Answer 1

Error arose due to referencing register button from activity_main.xml layout which is view of MainActivity , from RegisterActivity which has another register button in it's view of activity_register.xml

sorry to waste your time . the register button from activity_main thus wasn't initialised in the Register.java