This is still an issue with Xcode 12.1 GM Seed (12A7403)

We have gained some insights: This problem occurs on Big Sur only. The mapping model causing the above error contains a custom source fetch expression. When working with a mapping model in Xcode on Big Sur and trying to add or edit a custom source fetch expression Xcode crashes. Big Sur 11.0.1 (20B50) Xcode Version 12.4 (12D4e)

The problem still exists with Xcode 12.4 (12D4e) on macOS 11.2 (20D64)