No errror shown in android studio but still setOnClickListener works only on one button while on other button it does not work

Question

My java code:

public class StartActivity extends AppCompatActivity {

    private Button mRegBtn;
    private Button mLoginBtn;

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

        mRegBtn = (Button) findViewById(R.id.start_reg_btn);
        mLoginBtn = (Button) findViewById(R.id.start_login_btn);

        mRegBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                Intent reg_intent = new Intent(StartActivity.this, RegisterActivity.class);
                startActivity(reg_intent);
            }
        });

        mLoginBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                Intent login_intent = new Intent(StartActivity.this, LoginActivity.class);
                startActivity(login_intent);

            }
        });
    }
}

The LogCat shows this:

2020-04-30 12:53:42.391 31407-31407/com.example.chattting E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.chattting, PID: 31407
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.chattting/com.example.chattting.LoginActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.appcompat.app.ActionBar.setTitle(java.lang.CharSequence)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2947)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3012)
        at android.app.ActivityThread.-wrap12(ActivityThread.java)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1716)
        at android.os.Handler.dispatchMessage(Handler.java:110)
        at android.os.Looper.loop(Looper.java:232)
        at android.app.ActivityThread.main(ActivityThread.java:6802)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1103)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:964)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.appcompat.app.ActionBar.setTitle(java.lang.CharSequence)' on a null object reference
        at com.example.chattting.LoginActivity.onCreate(LoginActivity.java:43)
        at android.app.Activity.performCreate(Activity.java:6974)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2900)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3012) 
        at android.app.ActivityThread.-wrap12(ActivityThread.java) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1716) 
        at android.os.Handler.dispatchMessage(Handler.java:110) 
        at android.os.Looper.loop(Looper.java:232) 
        at android.app.ActivityThread.main(ActivityThread.java:6802) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1103) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:964) 

Answer 1

first answer i give. Based on your log, the line "Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.appcompat.app.ActionBar.setTitle(java.lang.CharSequence)' on a null object reference" says, that is having trouble setting an AppBar Title. My bet is that you have some sort of error, in one of the Activities you are trying to start, from the buttons. If you want more help, tell what is the button that is failing, and send the Other two activities code.