更新ListView会在ArrayAdapter上调用Null Pointer Exception

Question

尝试从互联网上获取的JSON字符串的ArrayList更新列表视图。

public void updateListView(ArrayList<String> crimes) {
        ArrayAdapter<String> crimeAdapter = new ArrayAdapter<String>(
                this.getActivity(),
                R.layout.listview_dbresults,
                R.id.listview_opendata,
                crimes);
        View rootView = inflater.inflate(R.layout.activity_main, container, false);

        ListView listView = (ListView) rootView.findViewById(R.id.dbResults);

        listView.setAdapter(crimeAdapter);
   }

@Override
    protected ArrayList<String> doInBackground(String[]... params) {
        // These two need to be declared outside the try/catch
        // so that they can be closed in the finally block.
        HttpURLConnection urlConnection = null;
        BufferedReader reader = null;

        // Will contain the raw JSON response as a string.

        try {
            URL url = new URL("JSON_LINK");
            Log.d("app", "url");

            // Create the request to OpenWeatherMap, and open the connection
            urlConnection = (HttpURLConnection) url.openConnection();
            urlConnection.setRequestMethod("GET");
            urlConnection.connect();

            // Read the input stream into a String
            InputStream inputStream = urlConnection.getInputStream();
            if (inputStream == null) {
                // Nothing to do.
                return null;
            }
            reader = new BufferedReader(new InputStreamReader(inputStream));
            ArrayList<String> crimes = new ArrayList<>();

            String line;
            while ((line = reader.readLine()) != null) {
                // Since it's JSON, adding a newline isn't necessary (it won't affect parsing)
                // But it does make debugging a *lot* easier if you print out the completed
                // buffer for debugging.
                crimes.add(line);
            }
            Log.d("app", "I've read all lines");
            return crimes;
        } catch (IOException e) {
            Log.e(LOG_TAG, "Error ", e);
            // If the code didn't successfully get the weather data, there's no point in attemping
            // to parse it.
            return null;
        } finally {
            if (urlConnection != null) {
                urlConnection.disconnect();
            }
            if (reader != null) {
                try {
                    reader.close();
                } catch (final IOException e) {
                    Log.e(LOG_TAG, "Error closing stream", e);
                }
            }
        }
    }

        protected void onPostExecute(ArrayList<String> crimes)
        {
                if (crimes.size() == 0 || crimes == null) {
                // Stream was empty.  No point in parsing.
                return;
            }
            DataFetcher dataFetcher = new DataFetcher();
            dataFetcher.updateListView(crimes);
            }

doInBackground从互联网URL获取JSON，然后将其传递给onPostExecute，然后从DataFetcher类（单独的类）调用该方法以更新该类中已有的现有列表视图。

java.lang.NullPointerException：尝试调用虚方法 ＆＃39; java.lang.Object android.content.Context.getSystemService（java.lang.String）＆＃39;在空对象引用上 在我的updateListView（）方法中发生，其中ArrayAdapter被实例化。