That .fig file is not the complete GUI, it is only the visible elements. The callback functions it references are not contained in the figure, so when you try to execute a function, that function can't work properly, because the guidata wasn't set correctly.
You have already identified the solution: don't start the GUI by opening the fig, but start it with the function. If you can't keep them apart, don't use a fig, but either click the 'generate code' option in GUIDE, or avoid GUIDE in the first place.
If you want some hints about avoiding GUIDE, I'dd be happy to give you some pointers about what I do (or you can just have a look at my FEX submission to see some examples).
