What is this and how do I fix it?

[Demgor]

I keep getting this error message when I start a battle. The back trainer sprite is missing.

[Pokémon Essentials version 21.1]
[v21.1 Hotfixes 1.0.9]

Exception: NoMethodError
Message: undefined method `width' for nil:NilClass

Backtrace:
Battle_Scene_BaseAnimation:105:in `ballTracksHand'
Battle_Scene_Animations:427:in `createProcesses'
Battle_Scene_BaseAnimation:12:in `initialize'
Battle_Scene_Animations:397:in `initialize'
Scene_PlayAnimations:122:in `new'
Scene_PlayAnimations:122:in `block in pbSendOutBattlers'
Scene_PlayAnimations:112:in `each'
Scene_PlayAnimations:112:in `each_with_index'
Scene_PlayAnimations:112:in `pbSendOutBattlers'
Battle_ActionSwitching:291:in `pbSendOut'

Is anyone able to help? I added new trainer sprites but I'm pretty sure they worked fine at first. I re-added the base sprites and that didn't help anything.