It happens too often last time.
<https://xojo.com/issue/50060>
It was happening for me every couple of hours with 17rX, but has gone away entirely with 18r1. Make sure you clear your Xojo cache (in ~/Library/Caches/Xojo/).
Cache clean doesn’t help for 2017R3
At one point there was a bug where if you closed a window while a thread was still running, a crash occurred. Could it be of the same nature ?
are you using any plugins or declares?
Using MBS. Crashed the Xojo itself
See if you can disable the functionality that uses the MBS plugins, I know it’s going to be hard, but most crashes.