Segfault when undoing tie over distance notes

GIT commit: d470cbd

Easily reproduced:

1. create a new document
2. hit n and insert three notes (ggg)
3. hit esc and ctrl+select the first and the third note with the mouse
4. click the tie button with the mouse
5. undo tie with crtl+z

Expected behavior: Three notes with no tie. Actual behavior: Segfault.