Standard music notation can appear in a guitar tablature staff, potentially causing segmentation faults.

Steps to reproduce (using revision ef3a7da, though any relatively recent version of Musescore should produce this):

1) Open any guitar pro file and select a guitar part.
2) Select a note in the standard notation staff.
3) Press Ctrl+Shift+Down to move it to the tab staff.

Result: The note will move down to the staff used to represent guitar tab. To get the segmentation fault:

4) Save and exit.
5) Open the file with Musescore.

Note that a similar problem is also possible by selecting a note in the guitar tab staff, then using Ctrl+Shift+Up to move it up the standard notation staff. If a user manages to move a note to the wrong staff accidentally, they will likely encounter the segmentation fault after they save, exit, and re-open the file later. I've attached an image demonstrating this issue.

I already have some idea on how to fix this, I'll make a PR with a solution later on when I'm done teaching for the day.