[Trunk-r5489] Adding instruments to empty sheet is not possible

I noticed this problem while using the OMR in MuseScore to import PDFs. In importpdf.cpp, i am able to get just the empty score (so without measures etc.) by commenting out everything between score->setOmr(omr) and score->setShowOmr(true). The problem is that it isn't possible to add instruments via Create->Instruments to this empty sheet.

I also tried it with this code: http://pastebin.com/vpPgps7h
This creates an empty sheet, but again, adding instruments via the 'Create -> Instruments' menu is not possible. They are added in the system (in the Staves overview of Create->Instruments) but not displayed on screen.

The problem does not appear when an existing sheet is opened and all instruments are removed. Then it is possible to add instruments to the empty sheet.

OS: Ubuntu 11.10