crash when attempting to 'load from score' for custom soundfonts in fluid

[mkj42]

This happened a lot a while ago, but I blamed my updated soundfonts at the time.
(I was adding expressive presets, so I soon discovered it was more reliable to add these new fonts from scratch and re-save the file.)
It just happened again today though, on a file I was working on completely successfully for the last few days.
I put off reporting this because I cannot readily reproduce it. It happens semi-rarely, but I figured I should report it.

Coincidence or not:
It may happen shortly after using 4.3.2 or 4.4 nightly, but again, maybe not every time.
I really can't be sure. Sorry about that.

And sometimes, it can crash repeatedly when trying to 'restore previous session', and then be fine.
#532