[quote=410028:@Derk Jochems]Some things might cause this like:
If you use addhandler on timers for example you have to removehandler and nil the timer before closing tue window or it will crash.[/quote]
If you can reproduce this in a small demo app and it hasn’t been put in already then this should be put in as a bug as the framework should be taking care of this.
Not sure if this helps, but I am having a bug in 2018 r3 64-bit MacOS debug and build which appears to be related to the windowclose event firing before lostfocus event fires or finishes leaving references to self in the lostfocus event (for example) being Nil which would usually cause a hard crash but would sometimes give a Nil object exception. The bug was not consistent and appeared to happen more frequently when you hit the close button on a window vs hitting a button which calls the close event (it happened either way but more frequently with the windowclose button). Disabling the windowclose button on the window only made the problem happen less but did not eliminate it.
Downgrading to 2017 r3 did stop the problem.
Actually not cause this can be in a thread. Not sure but i think i had this when a thread subclass had a timer, that had been activated fro the thread that got destructed before the timer fired the last time.
Ah, in a thread, that’s a different matter 
I have this crash too. It happens when I close a window containing TextFields, TextAreas, ListBox and buttons. Sometimes I open the window and close it and it crash. In the open event of the window I give the focus to a TextField, if I click in another TextField before close the window it doesn’t crash (or less often).
I use Xojo 2018r2 and El Capitan 10.11.6.