MediaPlayer crash. Attempt to invoke virtual method

Question

public class MainActivity extends AppCompatActivity {

MediaPlayer mp;

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

    mp = MediaPlayer.create(this, R.raw.crack);

    final ImageButton imageButton = (ImageButton) findViewById(R.id.nut);
    imageButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {

            mp.start();
        }
    });
}

This code gives me the following error:

FATAL EXCEPTION: main Process: -, PID: 9232 java.lang.NullPointerException: Attempt to invoke virtual method 'void android.media.MediaPlayer.start()' on a null object reference at -.MainActivity$1.onClick(MainActivity.java:48) at android.view.View.performClick(View.java:5226) at android.view.View$PerformClick.run(View.java:21266) at android.os.Handler.handleCallback(Handler.java:739) at android.os.Handler.dispatchMessage(Handler.java:95) at android.os.Looper.loop(Looper.java:168) at android.app.ActivityThread.main(ActivityThread.java:5845) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:797) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:687)

Does anyone know what's wrong with my code? I followed some tutorials and I thought I did it right but apparently not.