Check here https://github.com/The-Aether-Team/The-Aether/blob/04c87896d2d1a86f0350bf54d05a27562ba21808/src/main/java/com/aetherteam/aether/client/gui/screen/perks/MoaSkinsScreen.java#L412C231-L412C231

Actually this may not be the right source, it may be after this.

Nevermind, I should be ensuring MoaSkins.getMoaSkins().get(this.customizations.getMoaSkin()) != null in the check that the packet code is in.