无法在AsyncTask中未为ProgressDialog调用Looper.prepare（）

Question

我不明白为什么会收到此错误。我正在使用AsyncTask在后台运行一些进程。我有：protected void onPreExecute() {    connectionProgressDialog = new ProgressDialog(SetPreference.this);    connectionProgressDialog.setCancelable(true);    connectionProgressDialog.setProgressStyle(ProgressDialog.STYLE_SPINNER);    connectionProgressDialog.setMessage("Connecting to site...");    connectionProgressDialog.show();    downloadSpinnerProgressDialog = new ProgressDialog(SetPreference.this);    downloadSpinnerProgressDialog.setProgressStyle(ProgressDialog.STYLE_SPINNER);    downloadSpinnerProgressDialog.setMessage("Downloading wallpaper...");}当我doInBackground()根据情况进入时：[...]    connectionProgressDialog.dismiss();downloadSpinnerProgressDialog.show();[...]每当我尝试downloadSpinnerProgressDialog.show()我都会收到错误消息。有想法吗？