Describe the bug

I am not sure how and when this happened, but while doing some undo/redo I encountered an error, which led my Gaphor instance in a bad state (ie/ whatever I would do from there would trigger another error, even when trying to save my project).

Maybe the error was I was modifying a state machine (playing with the Entry attribute of a state), or when I tried creating a new class diagram, it's unclear.

To Reproduce

Unclear, but undo/redo related.

The error report is too long for the Github issue system, so here is it as a file:
Gaphor-logs-error.txt

Expected behavior

No error.

OS

• Linux − Fedora 39, GNOME
• macOS
• Windows

Version

Version of Gaphor: 2.23.2