使用异步任务时遇到一些问题

Question

  private class LongOperation extends AsyncTask<String, Void, String> {

    @Override
    protected String doInBackground(String... params) {
        for (int i = 0; i < 5; i++) {
            try {
            Thread.sleep(3000);
            } catch (InterruptedException e) {
                Thread.interrupted();
            }
        }
        return "Executed";
    }
    @Override
    protected void onPostExecute(String result) {
    AlertDialog.Builder bu=new AlertDialog.Builder(getApplicationContext());                                                              
        bu.setTitle("Sorry");
        bu.setMessage("Your time is up.   Score was:"+(score*5));
        bu.setPositiveButton("Exit", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialogInterface, int i) {
                    finish();
            }
        });
        AlertDialog a =bu.create();
        a.show();
    }
    @Override
    protected void onPreExecute() {}
    @Override
     protected void onProgressUpdate(Void... values) {}
   }

  

此活动需要使用Theme.AppCompat主题（或后代）。   这是我在onPostExecute（）方法上遇到的异常