按回或主页时应用程序停止工作？

Question

我有3个类，类扩展线程的GameThread，扩展Activity的类Main，扩展surfaceview的类GameView实现surfaceHolder.onCallback：

 public class Main extends Activity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    Log.i("oncreate","ONcreate");

     requestWindowFeature(Window.FEATURE_NO_TITLE);
     setContentView(R.layout.gameloopxml);
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}
}

类GameThread：

public class GameThread extends Thread {

GameView gameview;
SurfaceHolder surfaceHolder;
public boolean running;
       public GameThread(SurfaceHolder surfaceHolder,GameView gameview){

this.gameview=gameview;
running=false;
this.surfaceHolder=surfaceHolder;
    }

@Override
 public void run() {
Canvas c;
//super.run();
while(running){
    c=null;
  try{

c=surfaceHolder.lockCanvas();
 synchronized (surfaceHolder) {
     if(c!=null){
 c.drawColor(Color.BLACK);
 gameview.draw(c);
 gameview.update();
     }
 }


   }finally{
if(c!=null){
    surfaceHolder.unlockCanvasAndPost(c);
}
   }
   }


   }
   } 

类GameView：

      public class GameView extends SurfaceView implements SurfaceHolder.Callback{
GameThread gamethread;
Paint p=new Paint();
int x,y;
Bitmap bitmap;
public GameView(Context context, AttributeSet attrs) {
    super(context, attrs);

    Log.i("surface", "constructor");
    bitmap=BitmapFactory.decodeResource(getResources(), R.drawable.ic_launcher);

        //super(context);
    p.setColor(Color.RED);
        //add the callback this in order to intercept events
        getHolder().addCallback(this);
        //create the gameloop


        //make it foccusable so it can handle events
        setFocusable(true);


}

@Override
public void surfaceChanged(SurfaceHolder holder, int format, int width,
        int height) {
    Log.i("surface", "changed");
}

@Override
public void surfaceCreated(SurfaceHolder holder) {
    Log.i("surface", "created");
    gamethread=new GameThread(getHolder(),this);
    gamethread.running=true;
    gamethread.start();
}

@Override
public void surfaceDestroyed(SurfaceHolder holder) {
    Log.i("surface", "destroyed");
    boolean tryagain=false;
    while(tryagain){

        try {
            gamethread.join();//destroy the thread
            tryagain=false;
            gamethread.running=false;
        } catch (InterruptedException e) {
            //try again shutting down the thread
        }
    } 
}

public void draw(Canvas c) {
    c.drawRect(x, y, x+80, y+80, p);
    }
public void update(){
    x++;
    y++;
}
       }

我的代码中的一切都运行良好，只要按下后面或主页键我就会在logcat中获得Nullpointer异常。