This is a known issues. Xcode versions prior to Xcode 11.5 crash on launch on macOS 11.
Curretly there is one workaround for this. You've to run the following command in the Terminal app to avoid crashes when starting Xcode:
defaults write com.apple.dt.Xcode DVTDisableMainThreadChecker 1
Now deactivate "Main Thread Checker" in your Xcode project:
Edit Scheme ->
Uncheck Main Thread Checker.