Inventory Click Event Error

[mercurialmusic]

Hello! I'm getting this error in Spigot 1.14.3 using MagicArenas build #92: https://gist.github.com/mercurialmusic/8ab8c8e86476ba5bd5702bf8191184ad

The error doesn't seem to be affecting anything as far as I can tell yet. So far just console spam. It happens whether or not a player is in an arena.

Thanks!