Ok, so you picked a button, and when you click on "Prefill actions for..." dropdown nothing happens? Or it is blank? Or is that when it kicks you out?
The error to my untrained eyes seems to suggest you didn't select an "output" device in the Prefill Actions section.
As an alternative to Button Controller, I like Advanced Button Controller community app. I find it much easier, cleaner, and nicer to use. You might want to try it and see if it is a device issue (still doesn't work) OR whether Button Controller 3.1 is giving you trouble.
