As others have noted, this behavior also only seems to happen for me when using XCode on an external monitor, not if XCode is used on the MBP monitor. It seems like Big Sur has numerous little compatibility issues with external monitors (I was having bad flickering issues and it seemed to be a conflict with a Wacom tablet driver). I sort of wonder if it's a change they made in Big Sur to accommodate M1 as M1 can only handle one external (I have two). Funny thing is, it only crashes when clicking "Behaviors" when I have both externals connected, but I tried unplugging one external and using only one (so as if I had an M1) and it crashes on Preferences alone. I opened XCode and clicked Preferences > Behaviors to crash it a few times just to send a few reports hopefully they notice lol.