使用AsyncTask时出现不稳定的错误

Question

当我在网络wifi工作时，程序表现完美。但如果我在移动互联网网络中，我们经常会收到错误。它出现在不同时间的不同片段中，但总是在同一个地方。

这是我的AsyncTask：

public class MyAsincTask extends AsyncTask<String, Void, Document> {
    private Document document;
    private final ProgressDialog progressDialog;

    public MyAsincTask(Context context) {
        progressDialog = MyProgress.getProgressDialog(context);
    }

    @Override
    protected void onPreExecute() {
        progressDialog.show();
        super.onPreExecute();
    }

    @Override
    protected Document doInBackground(String... params) {
        try {
            document = Jsoup.connect(params[0]).get();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return document;
    }

    @Override
    protected void onPostExecute(Document document) {
        progressDialog.cancel();
        super.onPostExecute(document);
    }
}

所以我称之为：

 public void setData(){
        new MyAsincTask(getActivity()){
            @Override
            protected void onPreExecute() {
                count=0;
            }
            @Override
            protected void onPostExecute(Document document) {
                super.onPostExecute(document);
                guests = new ArrayList<>();
                Elements elementsUserId = document.select("guest_id");
                Elements elementsNumbers = document.select("room");



                for (int i = 0; i < elementsNumbers.size(); i++) {
                    GuestBean guest = new GuestBean();
                    guest.setUserId(elementsUserId.get(i).ownText());
                    guest.setRoom(elementsNumbers.get(i).ownText());                  
                    guests.add(guest);
                    count++;
                }
                if (getActivity()!=null) {
                    GuestsListAdapter adapter = new GuestsListAdapter(guests);

                    list.setAdapter(adapter);
                    progressDialog.cancel();
                    swipeLayout.setRefreshing(false);
                    header = "Гости " + hotel + "" + "(" + count + ")";
                    getActivity().setTitle(header);
                }
            }
        }.execute(link);
    }

当我打开碎片时有时会出现此错误。它发生在不同的片段，但始终在相同的区域代码中：

01-06 19:55:49.765  11237-11237/? E/AndroidRuntime﹕ FATAL EXCEPTION: main
    java.lang.NullPointerException
            at com.managment.pavel.managmentgradle.fragments.GuestsList$1.onPostExecute(GuestsList.java:100)
            at com.managment.pavel.managmentgradle.fragments.GuestsList$1.onPostExecute(GuestsList.java:91)
            at android.os.AsyncTask.finish(AsyncTask.java:602)
            at android.os.AsyncTask.access$600(AsyncTask.java:156)
            at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:615)
            at android.os.Handler.dispatchMessage(Handler.java:99)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:4464)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:511)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:822)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:589)
            at dalvik.system.NativeStart.main(Native Method)

第91行：

new MyAsincTask(getActivity()){

第100行

Elements elementsUserId = document.select("guest_id");