What's the best practice in Android about uploading a photo with a downloadable message?

I need to upload a photo to the server and show the uploading message.

I tried with this code, but ProcessDialog is unstable ...

new UploadTask(MainActivity.this).execute(bitmap);
....
private class UploadTask extends AsyncTask<Bitmap, Void, Long> {                       
   private ProgressDialog dialog = null;        
   private Context context;     
   public UploadTask(Context context){
     this.context = context;
   }        
   protected void onPreExecute() {      
        this.dialog = new ProgressDialog(this.context);
        this.dialog.setMessage("Loading...");
        this.dialog.setCancelable(false);           
        this.dialog.show();         
   }

 protected void onPostExecute(Long result) {                    
    if(this.dialog != null){
        this.dialog.dismiss();
    }
}