From Xojo Documentation
If a RuntimeException is not caught anywhere else (using Exception or Try...Catch), this event is called so you may handle the exception. Return True to suppress the quit behavior. Return False to use the default quit behavior.
If a RuntimeException is not caught anywhere else, this event is called so you may handle the exception. See the example.
This code in the UnhandledException event of the App class catches any unhanded exceptions and displays the name of the exception along with the error stack: