This does not happen in b108. And many mods are being affected by this, including MineMenu, which also doesn't happen in b108.

GirafiStudios/MineMenu#51

Crash log: https://hastebin.com/izuzufecec.txt

EDIT: Crashed when joining a server

Thanks for reporting, will look into it.

EDIT: The relevant Forge change: MinecraftForge/MinecraftForge@cdbd842

Fixed in v1.0.11: https://minecraft.curseforge.com/projects/appleskin/files/2698165