I was trying to Toast a message to show a debug message from a Fragment method that I had written a couple of days ago. I got the below error message when running the app.

java.lang.RuntimeException: Can't toast on a thread that has not called Looper.prepare()

What I didn’t realize was the method was run on a separate thread (as opposed to the main/UI thread) since it was doing an IO operation. There are multiple ways to handle this, but the most easiest of them is to do this.

activity!!.runOnUiThread( Runnable {
  Toast.makeText(activity, "Saving ${messages.size} messages", Toast.LENGTH_SHORT).show()
})