Thanks for taking the time to open a bug report!

Describe the bug
The error that appears in console appears when you interact with an item (from ItemJoin)
EDIT: This error appears everytime you close a inventory GUI, it could be the creative GUI, another Plugin's inventory GUI...

To Reproduce
Steps to reproduce the behavior:
0. Install ProtocolLib 1.8 and 1.8.8 spigot

1. Configure an item
2. Click on the item
3. See error

Expected behavior
The error does not appear because it spams console

Screenshots
If applicable, add screenshots to help explain your problem.

Pastebin
https://pastebin.com/Y9LaUBqU

Please complete the following information:

• Resource Version and Build Number: v5.2.3
• Server Version: Spigot-1.8.8

Additional context
EDIT: I noticed the problem is because of ProtocolLib 1.8, the problem is, I can not update ProtocolLib because I need that specific version.

Okay so I can confirm it is because of the Legacy protocol version.
The latest snapshot of ItemJoin should now fix this issue.

Link; https://ci.craftationgaming.com/job/ItemJoin/791/
Remember to remove your old ItemJoin.jar file

Let me know!~

Officially fixed in the latest release,

You can now download it here; https://www.spigotmc.org/resources/itemjoin.12661/download?version=451699