Blunt fix for null references errors.

I was seeing a NullReferenceError on the first line changed when previewing a model, closing the model editor, then repeating - on the third close it would throw the error.  (This was editing Project M's sc_selcharacter.pac > MiscData[30] > MenSelchrBack2_TopN, or MenSelchrFaceI_TopN - I didn't test any others.)

The second error (with _targetModels) would show up when the same thing was done, but would take between 2 and 7 iterations to crash.  It seemed to happen more consistently when editing was done - changing the animation for MenSelchrFaceI was what I was trying to do when I came across this.

Obviously this is a blunt fix; it would be better to learn why these are null sometimes.  I think _targetModels in particular should probably never be null, and things setting it to that should probably be making it be an empty list instead.