W/System.err: java.lang.NullPointerException: Can't toast on a thread that has not called Looper.prepare()

This error occurs when you try to show a Toast message on a thread that does not have a Looper prepared. The Looper is responsible for handling messages and events in Android, and it is required for showing Toast messages.

To fix this error, you need to make sure that you are showing the Toast message on the main UI thread, which has a Looper prepared by default. You can do this by using the runOnUiThread method, which runs your code on the UI thread.

Here is an example of how to show a Toast message on the UI thread:

runOnUiThread(new Runnable() {
   public void run() {
      Toast.makeText(getApplicationContext(), "Your message here", Toast.LENGTH_SHORT).show();
   }
});

Alternatively, you can also use a Handler to post a message to the UI thread's message queue:

new Handler(Looper.getMainLooper()).post(new Runnable() {
   public void run() {
      Toast.makeText(getApplicationContext(), "Your message here", Toast.LENGTH_SHORT).show();
   }
});

Both of these approaches will ensure that the Toast message is shown on the main UI thread, and that a Looper is prepared to handle the message.