Crash on changing a property of bracket in a part in Inspector
Steps to reproduce:
1. Create a new score.
2. Add some bracket to a staff.
3. Generate parts (File→Parts→New All→OK).
So you obtain something like this score: test.mscz.
4. Switch to a part (Piano in the given file).
5. Select the bracket.
6. In Inspector change the bracket's column two times. Note that after the first change bracket will lose selection, this may be related to the issue.
OS: Debian GNU/Linux 9 (stretch), Arch.: x86_64, MuseScore version (64-bit): 3.0.0, revision: b3feb3a