Android活动在切换应用程序时崩溃

将Android活动推到后台（按应用程序切换器或主页按钮）时，应用程序立即崩溃（“不幸的是，应用程序已停止。”）。

我的活动如下所示：

public class MyActivity extends Activity {
    MyView myView;

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

有问题的视图使用线程进行绘制：

public class MyView extends SurfaceView implements SurfaceHolder.Callback
{
    protected Engine engine;

    protected SurfaceHolder surfaceHolder;
    protected Context context;

    private PaintThread thread;

    void initView() {
        // Initialize our screen holder
        SurfaceHolder holder = getHolder();
        holder.addCallback( this);

        // Get screen size
        DisplayMetrics displayMetrics = new DisplayMetrics();
        ((Activity) getContext()).getWindowManager().getDefaultDisplay().getMetrics(displayMetrics);

        // Initialize engine
        engine = new Engine(displayMetrics.widthPixels, displayMetrics.heightPixels);
        engine.init(context);

        thread = new PaintThread(holder, context, new Handler(), engine);
        setFocusable(true);
    }

    public MyView(Context context, AttributeSet attrs, int defStyle){
        super(context, attrs, defStyle);
        this.context = context;
        initView();
    }

    public MyView(Context context, AttributeSet attrs){
        super(context, attrs);
        this.context = context;
        initView();
    }

    @Override
    public void surfaceChanged(SurfaceHolder arg0, int arg1, int arg2, int arg3) {}

    @Override
    public void surfaceDestroyed(SurfaceHolder arg0) {
        Log.i("app", "here!");
        boolean retry = true;

        thread.state = PaintThread.PAUSED;
        while (retry) {
            try {
                thread.join();
                retry = false;
            } catch (InterruptedException e) {}
        }
    }

    @Override
    public void surfaceCreated(SurfaceHolder arg0) {
        if (thread.state == PaintThread.PAUSED){
            thread = new PaintThread(getHolder(), context, new Handler(), engine);
            thread.start();
        } else {
            thread.start();
        }
    }
}

线程的工作方式如下：

public class PaintThread extends Thread {
    private SurfaceHolder surfaceHolder;
    private Handler handler;
    private Context context;

    private Engine engine;

    public int state = 1;

    public final static int RUNNING = 1;
    public final static int PAUSED = 2;

    public PaintThread(SurfaceHolder surfaceHolder, Context context, Handler handler,
                       Engine engine) {
        this.surfaceHolder = surfaceHolder;
        this.handler = handler;
        this.context = context;

        this.engine = engine;
    }

    @Override
    public void run() {
        long previousUpdate = System.nanoTime();
        long beforeTime;
        long passedTime;
        long accumulator = 0;
        long dt = 1000/60;

        while (state == RUNNING) {
            beforeTime = System.nanoTime();
            passedTime = TimeUnit.MILLISECONDS.convert(beforeTime - previousUpdate, TimeUnit.NANOSECONDS);
            previousUpdate = beforeTime;

            accumulator += passedTime;

            while (accumulator >= dt) {
                engine.update(dt);
                accumulator -= dt;
            }

            Canvas c = null;
            try {
                c = surfaceHolder.lockCanvas(null);

                synchronized (surfaceHolder) {
                    c.drawColor(Color.WHITE);
                    engine.draw(c);
                }
            } finally {
                if (c != null) {
                    surfaceHolder.unlockCanvasAndPost(c);
                }
            }
        }
    }
}

surfaceDestroyed当按下其中一个按钮时，在其方法的视图中将显示日志消息，但应用程序仍然崩溃，并显示以下消息：

java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法“ void android.graphics.Canvas.drawColor（int）”

受影响的行来自PaintThread其run方法：

c.drawColor(Color.WHITE);

似乎lockCanvas在那时还没有返回我们可以使用的Canvas。这似乎是预期的行为，因为在按下这些按钮之一时将画布置于背景中。

但是，这一切都发生只是线程暂停之前。发生这种情况的设置是否有问题，或者如何防止这种情况发生？