"Maybe a race condition where "untitled" is closed shortly before/after the editor-notify signal is sent?"
That's in an edit I added to the first comment. The deleted comment was when I thought the problem was fixed by a PR, but it isn't. The plugin had stopped loading.
Did you connect to the editor notify signal "after"?
That does it. Changing "after" to false doesn't segfault. Changing back to true segfaults again.