从小部件的onDeleted方法调用handler.removeCallbacks会引发nullpointerexception

当用户删除窗口小部件时，我需要停止处理程序，但是调用handler.removeCallbacks会从onDeleted方法中引发nullpointerexception.我尝试了其他变通方法，例如在实现runnable的类中创建方法来杀死该runnable，但这也会引发nullpointerexception.
也许在调用onDeleted方法之后处理程序会变为null，所以我尝试将其放在onDisabled方法中，但没有停止.
我在做什么错了?

I need to stop the handler when the widget is removed by the user but calling handler.removeCallbacks throws a nullpointerexception from the onDeleted method. I tried other workarounds like creating a method,in a class which implements runnable, to kill the runnable but this throw a nullpointerexception also.
Maybe handler gets null after the call of the onDeleted method so I tried to put it in the onDisabled method but nothing stop.
What am I doing wrong?

代码在这里:

public class RAMWidget extends AppWidgetProvider {

private PieGraph pg;
private Context context;
private RemoteViews remoteViews;
private AppWidgetManager appWidgetManager;
private ComponentName widget;
private Handler handler;
private CustomRunnable runnable;

@Override
public void onUpdate(Context context, AppWidgetManager appWidgetManager, int[] appWidgetIds)
{
    this.context=context;
    this.appWidgetManager=appWidgetManager;
    remoteViews=new RemoteViews(context.getPackageName(),R.layout.widget_ram);
    widget=new ComponentName(context,RAMWidget.class);
    new DrawTask().execute();
    handler=new Handler();
    runnable=new CustomRunnable();
    handler.postDelayed(runnable,3000);
}

private class CustomRunnable implements Runnable
{
    private boolean stop;

    public CustomRunnable()
    {
        stop=false;
    }

    @Override
    public void run()
    {
         new DrawTask().execute();
         Log.i("STOP",stop "");
         if(!stop)
            handler.postDelayed(this,3000);
         else
            return;
         Log.i("STOP",stop "");
    }

    void killThread()
    {
        stop=true;
    }
}

private class DrawTask extends AsyncTask<Void,Void, Void>
{
    private PieSlice slice,_slice;
    private long total=0,free=0,rate=0;

    @Override
    protected Void doInBackground(Void... unused)
    {
        RandomAccessFile reader=null;
        try
        {
            reader=new RandomAccessFile("/proc/meminfo","r");
            long[] mems=new long[4];
            for(int i=0;i<4;i  )
            {
                String load = reader.readLine();
                String[] toks = load.split(":");
                mems[i] = Long.parseLong(toks[1].replace("kB","").trim());
            }
            total=mems[0]/1024;
            free=(mems[1] mems[2] mems[3])/1024;
            rate=(int)((float)(total-free)/total*100);
        }
        catch (Exception e)
        {
            e.printStackTrace();
        }
        if(reader!=null)
            try
            {
                reader.close();
            }
            catch (IOException e)
            {
                e.printStackTrace();
            }
        slice=new PieSlice();
        slice.setTitle("Available RAM");
        slice.setColor(Color.parseColor("#99CC00"));
        slice.setValue(total-free);
        _slice=new PieSlice();
        _slice.setTitle("Used RAM");
        _slice.setColor(Color.parseColor("#FFBB33"));
        _slice.setValue(free);
        publishProgress();
        return null;
    }

    @Override
    protected void onProgressUpdate(Void... values)
    {
        pg=new PieGraph(context);
        pg.measure(200,200);
        pg.layout(0,0,200,200);
        pg.setDrawingCacheEnabled(true);
        pg.addSlice(slice);
        pg.addSlice(_slice);
        pg.setInnerCircleRatio(150);
        for (PieSlice s : pg.getSlices())
            s.setGoalValue(s.getValue());
        pg.setDuration(1000);
        pg.setInterpolator(new AccelerateDecelerateInterpolator());
        pg.animateToGoalValues();
        pg.setPadding(3);
        remoteViews.setTextViewText(R.id.widget_ram_text, "Total RAM "   total   " MB");
        remoteViews.setTextViewText(R.id.widget_ram_text1,"Avaiable RAM " (total-free) " MB");
        remoteViews.setTextViewText(R.id.widget_ram_text2,"Used RAM " free " MB");
        Bitmap bitmap=pg.getDrawingCache();
        Canvas canvas = new Canvas(bitmap);
        Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);
        paint.setColor(Color.BLACK);
        paint.setTextSize(18);
        paint.setShadowLayer(1f,0f,1f,Color.WHITE);
        Rect bounds=new Rect();
        paint.getTextBounds(rate " %",0,new String(rate " %").length(),bounds);
        int x=(bitmap.getWidth()-bounds.width())/2;
        int y=(bitmap.getHeight() bounds.height())/2;
        canvas.drawText(rate " %",x,y,paint);
        remoteViews.setImageViewBitmap(R.id.graph_widget,bitmap);
        appWidgetManager.updateAppWidget(widget,remoteViews);
    }
  }

  @Override
  public void onDeleted(Context context, int[] appWidgetIds) {
      runnable.killThread();
      handler.removeCallbacks(runnable);       //both of them don't work
      super.onDeleted(context, appWidgetIds);
  }

  @Override
  public void onDisabled(Context context) {
      runnable.killThread();
      handler.removeCallbacks(runnable);
      super.onDisabled(context);
  }
}