I've been just trying to learn APE, and every single time I try to add a normal map to the material I'm modding, APE crashes, and then keeps crashing until I delete the texture from the texture_assets folder. EVENTUALLY after more crashes it starts working again.
I made the normal map from this site, if anyone's wondering
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EDIT: I just figured out that literally selecting any material crashes APE. What a quality tool by Treyarch
EDIT 2: FOUND THE ANSWER! Keep your images in powers of 2 (usually 1024x1024) or else APE freaks out!