As it turns out, fixing that error has now caused a new one.
[Pokémon Essentials version 20.1]
[v20.1 Hotfixes 1.0.0]
Exception: NoMethodError
Message: undefined method `maps' for nil:NilClass
Backtrace:
[Following Pokemon EX] Event Sprite.rb:44:in `refresh'
050:Game_FollowerFactory:336:in...