Fix crash when trying to difference merge w/o third model

[fortypercnt]

While difference merging is supposed to use three models according to the formula, it is probably preferable to have this operation also succeed without specifying a third model rather than crashing the UI and requiring a restart.

[DarioFT]

Hey, I've fixed all related problems regarding memory usage on model merging and the related UI and app crashes in PR #4151

Maybe @AUTOMATIC1111 can take a look and check if it can be merged.

[cluder]

i tried to test it, but in my tests the resulting model did produce the same images as model A. i don't know about the merging variants, but if it dosen't make sense to use additive merging, without a 3rd model, it might be better to just create a warning e.g. "please select Tertiary model" instead of continuing to merge.

Otherwise the fix works and no exception is thrown. so the question is, does it make sense to merge 2 models by 'add difference'? (or does the merge code actually work with 2 models)

[cluder]

i just saw that #4151 did add the message "Select a tertiary model (C) or consider using 'Weighted sum'" when trying without the 3rd model, so i think 4151 would be the better solution than this pr.

[fortypercnt]

Agree, closing this since #4151 is a better solution.