Describe the bug
Every time when i m closing the minecraft server i m getting this error :) nothing more. But the plugin works well without any problem.

To Reproduce
Steps to reproduce the behavior:

1. items.yml -> https://pastebin.com/57FRJQKp
2. config.yml -> https://pastebin.com/a3ub8FsC
3. Close the server

Screenshots
https://imgur.com/a/DEszSZO

Please complete the following information:

• Resource v5.1.0-RELEASE-b545
• Server: 1.16.4 1.16.4-R0.1-SNAPSHOT

I am 99% sure this issue is already resolved in the latest snapshot as it looks fairly familiar.
Link; https://ci.craftationgaming.com/job/ItemJoin/597/
Remember to remove your old ItemJoin.jar file

This bug has been officially fixed in the release of ItemJoin v5.2.0.
You can grab it here; https://www.spigotmc.org/resources/itemjoin.12661/download?version=383801

Please read the changelog as the documentation has not been updated yet;
https://github.com/RockinChaos/ItemJoin/wiki/Recent-Changes