MuseScore crashes when trying to open a no longer existing score

1. Build MuseScore yourself, using a Debug setup (like using QtCreator/MinGW), using recent code from the master branch
2. Set up MuseScore to restore last session
3. Start it and open some score
4. Close it and delete that score
5. Start it again
6. A dialogbos shows complaining aboutz a score that can't be found
7. Continue

-> crash , due to a failed assertion:

Critical: "ASSERT FAILED!": data.isValid() in ...mscore/scoretab.cpp 1line 71

Workaround: revert to factory settings or place a score with the same name at the same place.

Won't crash in and non-Debug build, as there the assertion evaluate to a NOP

Happens since 0b0ea52d01