crash on undo of line break action

Windows 7, GIT commit: 3e51505

Reproducible in the attached test file, not in simpler examples

1) load attached file
2) view violin part
3) delete line break at end of third system
4) undo

Result: crash

Really, lots of line break operations are problematic in this file, probably having something to do with the multimeasure rests. It's possible the file is corrupt in some sense at this point from my attempts at working around #18510: incorrect barlines with start repeat at beginning of system. But anyhow, the above steps crash every time. I can't get QtCreator to tell me anything useful.