Android : Showing progress notification from library project android

I want to develop a download project library for my android applications. I created a singleton class that takes an object -including url of file that should be download and a compactNotificationBuilder object - and then add it to the download queue. When download start it will show the progress into the notification bar.

I use below code in my library:

    public void initialize(Context context)
    {
            //------> set other private variable
            this.appContext= context;
            this.applicationPackageName= context.getApplicationContext().getPackageName();
            mNotifyManager = (NotificationManager) appContext.getSystemService(Context.NOTIFICATION_SERVICE);

}

private void downloadFile(DownloadToken token){
        try{

            this.mBuilder= token.getnCompactBuilder();
            new Downloader().execute(token.getTargetUrl());

        }catch(Exception ex){
            ex.printStackTrace();
        }
    }

    private class Downloader extends AsyncTask<String, Integer, Integer> {

        @Override
        protected void onPreExecute() {
            super.onPreExecute();

            // Displays the progress bar for the first time.
            mBuilder.setProgress(100, 0, false);
            mNotifyManager.notify(id, mBuilder.build());
        }

        @Override
        protected void onProgressUpdate(Integer... values) {
            // Update progress
            mBuilder.setProgress(100, values[0], false);
            mNotifyManager.notify(id, mBuilder.build());
            super.onProgressUpdate(values);
        }

        @Override
        protected Integer doInBackground(String... params) {

            int count;

            try {

                URL url = new URL(params[0]);
                URLConnection conection = url.openConnection();
                conection.connect();

                // getting file length
                int lenghtOfFile = conection.getContentLength();

                // input stream to read file - with 8k buffer
                InputStream input = new BufferedInputStream(url.openStream(), 8192);

                // Output stream to write file
                String path= Environment.getExternalStorageDirectory().getPath() + "/downloadedfile.file";
                OutputStream output = new FileOutputStream(path );

                byte data[] = new byte[1024];

                long total = 0;

                while ((count = input.read(data)) != -1) {

                    total += count;

                    // publishing the progress....
                    // After this onProgressUpdate will be called
                    publishProgress((int)((total*100)/lenghtOfFile));

                    // writing data to file
                    output.write(data, 0, count);
                }

                // flushing output
                output.flush();

                // closing streams
                output.close();
                input.close();

            } catch (Exception e) {
                Log.e("Error: ", e.getMessage());
            }

            return null;
        }

        @Override
        protected void onPostExecute(Integer result) {
            super.onPostExecute(result);

            //change text of progress bar
            mBuilder.setContentText("Download complete");

            // Removes the progress bar
            mBuilder.setProgress(0, 0, false);
            mNotifyManager.notify(id, mBuilder.build());

            //remove head item from download queue
            downloadQueue.remove(0);
        }
    }

}

and this code in my target project:

 protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    setContentView(R.layout.activity_main);
    Button b= (Button) findViewById(R.id.downloadBtn);
    b.setOnClickListener(this);
    DownloadManager.getInstance().initialize(this);        
}

@Override
public void onClick(View arg0) {
    //creating notification intent
    PendingIntent pendingIntent = PendingIntent.getActivity(
             this,
             0, 
             new Intent(this, MainActivity.class),
             0
             );

    //creating notification builder
    NotificationCompat.Builder mBuilder = new NotificationCompat.Builder(this);
    mBuilder.setContentTitle("mytitle")
            .setContentText("mytext")
            .setSmallIcon(R.id.icon)
            .setContentIntent(pendingIntent);

    //creating download token
    DownloadToken token = new DownloadToken(
            "myurl",
            mBuilder
            );

    //adding download token to download list
    DownloadManager.getInstance().addToQueue(token);

}

but when i run the app it crash on the onPreExecute method and line mNotifyManager.notify(id, mBuilder.build());

actually it open a class named ActivityThread and highlighted this line:

 Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "broadcastPackage");

does any body have any idea for fixing it?