Crash if exporting empty score

66e5ef9 on Win XPpro SP3 crashes when exporting an empty score (e.g. as .mxl)
Yes, this is a bit of a corner case and the answer might be: "Don't do it then", but here it happens in batch- or command line mode, attempting to convert some Capella files, a few of which seem to be 2.x format and as such not supported, so MuseScore emits a messages about that, which is fine, but right after that it tries to save the now empty score, and this crashes MuseScore, leaving behind a 0 byte export file.

I'd exect it a) not to crash and b) not to create an output file at all.

Can also be done interactively:
just create an empty score (take an existing one and delete every measure and frame)
try to export it as .mxl or .xml, probably any other format too.

You can also cause a crash, with the same empty score, when trying to access the page settings, probably the same cause.