I got this error with Xcode 12.4. Interestingly, I only get the crash when trying to access the Behaviors tab when Xcode is on my Thunderbolt Display (connected via the lightning connector to a lightning to USB-C adaptor). I've tested this both on my 2019 MBP (via the adapter) and on an older 2015 MBP (no adapter needed). Both MBP are running macOS 11.1. However (and this is where things get spicy) I have a third monitor connected (an LG, via a USBC to HDMI). I am able to open the Behaviors tab on this monitor just fine. Adding some habanero to the mix is that you can open the Behaviors on a save screen (for me its the MBP main monitor or my LG) then drag Xcode to the Thunderbolt Display (the crashing screen). This leaves the preference pane on the safe screen. Xcode doesn't crash until I drag the preference panel over to the crashing screen. As soon as that happens, poof! Xcode crashes... Like magic...!