Android Toast Messages not working

You're trying to display a Toast in a background thread. You should do all your UI operations on the main UI thread.

The exception RuntimeException: Can't create handler inside thread that has not called Looper.prepare() can be a little cryptic for beginners but essentially it tells you that you're in a wrong thread.

To solve it, wrap the toast to e.g. runOnUiThread():

activity.runOnUiThread(new Runnable() {
  @Override
  public void run() {
    Toast.makeText(...).show();
  }
});