MusicXML export: Insert check against empty "sections" list

[uliska]

I don't know what this code actually is about, but adding this
check made a file compile that before caused a traceback because of
trying to pop from an empty list.

@PeterBjuhr Please have a look into this and tell me if I'm curing the right symptoms with that.
Actually I don't have any clue what this sections object is for.

Addresses #76

[PeterBjuhr]

This shouldn't happen so there's a risk that the output will be bad anyway. And for that reason I would prefer try/except over if. (Also some would argue that it's more pythonic :)

[uliska]

Can you tell me what that "sections" actually refers to?

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[PeterBjuhr]

In short it keeps tracks of different sections of music in the source. It works as a stack with the active section on top. When the section ends it is removed from the stack. And there should never be removed more sections than are added.

[uliska]

Well, it did happen, and after that check it seemed to produce a file.
I didn't have the opportunity to check the results, though.

But do I understand you correctly that as this happened it more likely pointed to an issue elsewhere that I should investigate more closely?

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[PeterBjuhr]

Of course I didn't mean to say that it can't happen. But as you say it's an indication that something went wrong somewhere. Therefore to my mind also an exception is more suitable.