Up and Down buttons available in instrument selection after removing staff

1. 'Create New Score'.
2. At the instrument step, add piano.
3. Click on stave.
4. 'Remove'.
5. Click on remaining stave.
6. 'Add Staff'.
7. 'Remove'.

Result: 'Up' is available to click, to no effect.

8. Click on remaining stave.

Expected result: 'Up' and 'Down' disabled if there is one stave in an instrument.
Actual result: One after the other, I'm able to press 'Up' and 'Down' to no effect.

Using MuseScore 2.0 Nightly Build (279c5e4) - Mac 10.7.4.