Template didn't really fit this

So I'm running Wearable Backpacks with Carry On on a server. I forgot how to equip backpacks and used Carry On instead. The texture was a square peach colour. I quickly tried to put it back down but upon doing so my game crashed to desktop (server did not crash). On my client I got this:
https://ghostbin.com/paste/yq947
Which seems to be thrown by wearable backpacks. Not 100% sure what's causing the index out of bounds error, guessing its a state change from Carry On which is why I posted it here.

Version of Minecraft, Carry On, Forge

Minecraft 1.12.2
Carry On 1.12
Wearable Backpacks 3.1.3
Forge 14.23.5.2772

Any help appreciated.