This has happened to me quite a few times in different occasions.
But this time i can reproduce it so I thought to share with you guys.
Here’s what happens:
I copy a glyph with components and corners to a mask. When I decompose one of components and try do Undo this message pops up.
I can’t reproduce it since I quit glyphs last time.
It crashed once though while trying to reproduce it.
I’ve sent you the crash report.
I’m sending a screenshot anyway.