Crash on deleting last instrument

MuseScore crashes if you delete the only remaining instrument (staff) in the score. The same situation also occurs often (but not always) when deleting the topmost instrument (or staff) from a score.

Steps to reproduce:

1. Create a new score with one instrument (or use the demo score)
2. Open Create -> Instruments...
3. Select the instrument on the right and click the "Remove" button
4. Click OK to close the dialog

Expected behaviour: The dialog closes, and the removed staff (or staves) are removed from the score, leaving a blank page, except for titles and other text not directly related to the staff.

Actual behaviour: MuseScore crashes immediately upon clicking OK, throwing up a C++ runtime error.