Thanks for your replies! Bug is filed: FB8910801 The NSURL workaround actually works, the temporaryResourceValue not.

I got some peace on my private Mac by downloading the "Additional Tools for Xcode 12" and use Utilities/CrashReporterPrefs to set Crash Reporter to the "Basic" mode.

I get that error on my work machine each time I change any project settings. Clean project and recompile / run fixes it for me (even though it´s a PITA as it takes 3 minutes for the project I work on)

I have also some simulator processes constantly crashing - Xcode 12 with CalendarWidget et al, Xcode 11 some internal daemons - but I guess this is another issue than for the OP. Wiping with xcrun simctl delete all and re-instantiating runtimes doesn't help.

The place to report bugs is feedbackassistant.apple.com. As this seems to be related to the swift implementation, the swift project has a bug tracker at bugs.swift.org