I do have this exact same problem. I tried this on both Xcode 12.2 (12B45b) and 12.3 beta (12C5020f).
I tested this on Macbook Pro (2020, 32 GB RAM) Open Xcode on external monitor
Open Preferences tab -> Crashed
Open Xcode again on external monitor, dragged it to build-in monitor (macbookpro)
Open Preferences tab -> OK
Drag Xcode back to external monitor
Open preferences tab -> Crashed
??!!!
Please fix. Thank you.