When swapping items in it multiple times on a server, it sometimes disconnects the player with an error, but also sometimes completely crashes the server.
I've attached a pastebin with the crash thread dump

https://pastebin.com/ydysDcim

Here's the error message when the player disconnects with an error:
https://media.discordapp.net/attachments/1162742481478226031/1196916566655246468/JQMSEVH.png