AsyncTask Android generating NullPointerException

I have this static class in an Activity where its function and download a file and save it on the user's device. I tested it on a real device in Android version 7.1.1 and 4.1.2 and it worked correctly. However, by launching the application I have received error reports from Android versions 7.1, 6.0 and 7.0 referring to the NullPointerException of the InBackground process in the line referring to catch (Exception e) p>

Class Code

private static class DownloadFileFromURL extends AsyncTask<String, String, String> {

    private WeakReference<DownloadActivity> activityReference;

    DownloadFileFromURL(DownloadActivity context) {
        activityReference = new WeakReference<>(context);
    }

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        DownloadActivity activity = activityReference.get();
        activity.txt_status_progress.setText(String.format(activity.getString(R.string.txt_baixando_dados),String.valueOf(0)));
    }

    @Override
    protected String doInBackground(String... f_url) {

        DownloadActivity activity = activityReference.get();

        int count;

        try {
            URL url = new URL(f_url[0]);
            URLConnection conection = url.openConnection();
            conection.connect();

            int lenghtOfFile = conection.getContentLength();

            InputStream input = new BufferedInputStream(url.openStream(), 8192);

            FileOutputStream output = activity.openFileOutput(FOLDER + File.pathSeparator + FILE_SQL, Context.MODE_PRIVATE);

            byte data[] = new byte[1024];

            long total = 0;

            while ((count = input.read(data)) != -1) {
                total += count;
                publishProgress("" + (int) ((total * 100) / lenghtOfFile));
                output.write(data, 0, count);
            }
            output.flush();
            output.close();
            input.close();

        } catch (Exception e) {
            progressBar.setVisibility(View.GONE);
            txt_status_progress.setText(R.string.txt_erro_config);
        }
        return null;
    }

    protected void onProgressUpdate(String... progress) {
        DownloadActivity activity = activityReference.get();
        activity.txt_status_progress.setText(String.format(activity.getString(R.string.txt_baixando_dados),progress[0]));
    }

    @Override
    protected void onPostExecute(String file_url) {
        DownloadActivity activity = activityReference.get();
        new DownloadActivity.insertFromFile(activity).execute();
    }
}

What might be causing this error?