nullpointerexception in java on a null object reference

Question

For my android app, I get the error log below in. It is a NullPointerException. I am new to programming and I cannot figure out how to fix this error.

Error log:

2020-07-21 20:00:48.398 9683-9683/? E/SchedPolicy: open of /dev/cpuctl/bg_non_interactive/tasks failed: No such file or directory
2020-07-21 20:00:49.021 9683-9683/com.example.myapplication E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.myapplication, PID: 9683
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.myapplication/com.example.myapplication.StartActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2861)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2943)
        at android.app.ActivityThread.-wrap11(Unknown Source:0)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1630)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6626)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:811)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at com.example.myapplication.StartActivity.onCreate(StartActivity.java:37)
        at android.app.Activity.performCreate(Activity.java:7032)
        at android.app.Activity.performCreate(Activity.java:7023)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1236)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2814)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2943) 
        at android.app.ActivityThread.-wrap11(Unknown Source:0) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1630) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:164) 
        at android.app.ActivityThread.main(ActivityThread.java:6626) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:811) 

My code in android studio:

public class StartActivity extends AppCompatActivity {
    Button mlogin, mregister;
    FirebaseUser firebaseUser;

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

        firebaseUser = FirebaseAuth.getInstance().getCurrentUser();

        if (firebaseUser != null) {
            startActivity(new Intent(StartActivity.this, MainActivity.class));
            finish();
        }
    }

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

        mlogin = findViewById(R.id.login);
        mregister = findViewById(R.id.register);

        mlogin.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                startActivity(new Intent(StartActivity.this, LoginActivity.class));

            }
        });

        mregister.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                startActivity(new Intent(StartActivity.this, RegisterActivity.class));
            }
        });
    }
}

My layout activity code
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="center"
    android:padding="12dp"
    android:orientation="vertical"
    tools:context=".StartActivity">

    <ImageView
        android:layout_width="wrap_content"
        android:layout_height="50dp"
        android:src="@drawable/logo"/>
    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Log in"
        android:id="@+id/login"
        android:layout_marginTop="15dp"
        android:textColor="@color/colorPrimary"
        android:background="@drawable/button_background"/>
    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Register"
        android:id="@+id/register"
        android:layout_marginTop="15dp"
        android:textColor="@color/colorPrimary"
        android:background="@drawable/button_background"/>

</LinearLayout>

Answer 1

The error indicates that the variable mlogin or mregister are null.