I am a beginner in Android development and I'm getting this error that prevents my Asynctask from working using the UI:

```
java.lang.RuntimeException: An error occured while executing doInBackground()
        at android.os.AsyncTask$3.done(AsyncTask.java:300)
        at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:355)
        at java.util.concurrent.FutureTask.setException(FutureTask.java:222)
        at java.util.concurrent.FutureTask.run(FutureTask.java:242)
        at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
        at java.lang.Thread.run(Thread.java:841)

```

I can't understand why. I'm using Android Studio. Before building it for release, my app worked fine but now it crashes. This is my class:

```
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    pageNumber = getArguments().getInt(ARGUMENT_PAGE_NUMBER);
}

public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.activity_my, null);
    lv = (ListView) view.findViewById(R.id.listView1);
    adapter = new ArrayAdapter<String>(getActivity(), R.layout.list_item, R.id.product_name, titleList);
    new NewThread().execute();
    return view;
}
public class NewThread extends AsyncTask<String, Void, String> {
    @Override
    protected String doInBackground(String... arg) {
        Document doc;
        try {
            doc = Jsoup.connect("http://ot76.ru/mob/list.php?vt=" + (pageNumber + 1) + "&nl=0").get();
            title = doc.select("a");
            for (Element titles : title) {
                titleList.add(titles.html().replace("&nbsp;", "(") + ")    ");
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
        return null;
    }
    @Override
    protected void onPostExecute(String result) {
        lv.setAdapter(adapter);
        lv.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            public void onItemClick(AdapterView<?> parent, View view,
                                    int position, long id) {
                String str = lv.getItemAtPosition(position).toString();
                Intent intent = new Intent(getActivity().getApplicationContext(), Rasp.class);
                intent.putExtra("url", "http://ot76.ru/mob/getroutestr.php?vt=" + (pageNumber + 1) + "&nl=0&nmar=" + str.substring(0, str.indexOf(' ')).replace("к", "k").replace("а", "a").replace("б", "b").replace("с", "c").replace("д", "d"));
               intent.putExtra("stops", lv.getItemAtPosition(position).toString().substring(str.indexOf(' '), str.length() - 1));
                startActivity(intent);
            }
        });
    }
}

```