Paper 1.19.2:Error

[OnlyMatty]

I got an error while editing an NPC boss. The purpose of the boss was guarding a house in overworld from monsters. When I wanted to edit the Boss's potion effects, it closed the menu and some error popped up in my server console.

So I wanted to ask: What does the error mean and what is the problem that caused the error to show up?

The error log: https://pastebin.com/AZtBkA1c