Sometimes when player opens bosshop menu with itemjoin item this error shows in console.
Thanks for your awesome support.

ItemJoin 4.6

CraftBukkit version git-Spigot-7d78b81-d5e7885 (MC: 1.11.2) (Implementing API version 1.11.2-R0.1-SNAPSHOT)

3.) What is the problem that you are having?
Config: https://pastebin.com/x2uDKfSe
Error: https://pastebin.com/hNHqPVtN

This error has been noted to be fixed in the latest snapshot, it is completely safe to use and only contains this fix; http://ci.craftationgaming.com/job/ItemJoin/98/artifact/target/ItemJoin-4.8-SNAPSHOT.jar

ItemJoin v4.8 has been officially released containing this bug fix for official usage.
You can grab it here; https://www.spigotmc.org/resources/itemjoin.12661/download?version=223327

Please read the changelog as the documentation has not been updated yet;
https://www.spigotmc.org/wiki/recent-changes