[trunk] Crash on startup if no navigator window

Setup: trunk SVN rev. 4682; Qt lib 4.7.3; Ubuntu 11.04

Steps:

1) Start MuseScore
2) Select "Edit | Preferences..."
3) Uncheck "Show Navigator"
4) Close MuseScore
5) Re-start MuseScore

Result: MuseScore crashes during the start up.

Notes:

1) This seems to happen because the global _navigator object exists but its widget does not, so the statements
    static_cast<Navigator*>(_navigator->widget())->setScore(...);
in functions MuseScore::setCurrentScoreView(ScoreView* view), line 1333 and 1360 of file mscore/mscore/musescore.cpp trigger a SIGFAULT.

2) The attached patch also checks for the widget to exists in the tests right before those two lines.