MediaPlayer崩溃。尝试调用虚拟方法

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();
        }
    });
}

此代码给出了以下错误：

致命的例外：主要                                                                          过程： - ，PID：9232                                                                          java.lang.NullPointerException：尝试调用虚方法＆＃39; void android.media.MediaPlayer.start（）＆＃39;在null对象引用上                                                                              at -.MainActivity $ 1.onClick（MainActivity.java:48）                                                                              在android.view.View.performClick（View.java:5226）                                                                              在android.view.View $ PerformClick.run（View.java:21266）                                                                              在android.os.Handler.handleCallback（Handler.java:739）                                                                              在android.os.Handler.dispatchMessage（Handler.java:95）                                                                              在android.os.Looper.loop（Looper.java:168）                                                                              在android.app.ActivityThread.main（ActivityThread.java:5845）                                                                              at java.lang.reflect.Method.invoke（Native Method）                                                                              在com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run（ZygoteInit.java:797）                                                                              在com.android.internal.os.ZygoteInit.main（ZygoteInit.java:687）

有谁知道我的代码有什么问题？我遵循了一些教程，我认为我做得对，但显然没有。