Apple has fixed it, works as intended on macOS 11.5 Beta. Bug #FB8955055

I've got a feedback reply from Apple asking to re-check it on macOS 11.4 GM. Can't check it there however the issue is definitely resolved on macOS 11.5 Beta. Hopefully they will merge this fix later. It sucks that I had to re-do whole app without Lists...

Hello @Tylor, It works as intended on iOS. It crashes on macOS. Did you try that on a macOS target?

Filed a bug report #FB8965816.

The crash remains if there are children in items and you delete the last item.

Exactly the same code works as intended on iOS.