[trunk] Crash on opening the clef or time sig palettes

Tested with rev. 3305, Ubuntu 10.04, Qt SDK 10.02

Steps:

1) Create a simple, one part, score, selecting an instrument of your choice and accepting all other defaults.
2) Once the score is created, click on the "Clefs" or on the "Time Signatures" palette headings to open either palette:

==> the program crashes in Staff::useTablature() function (in staff.cpp, line 452) which is called with a this = 0 (!).

M.