[1.0] Crash when closed score while staff name in edit mode

Steps to reproduce bug
1. Open "bach-bc2" demo score
2. Double click the word "Trumpet" to edit the staff name
3. File > Close

--> Crash

MuseScore version: Tested using 0.9.6.3

Discussion: I tagged the title with 1.0 to make sure it is fixed before release. I don't have a build system set up to test the stable branch (since it uses a different Qt version).

(Operating System: Windows 7)