Information

Environment information
Plugin + server version info:

plugin is version 1.16.5
server is version 1.16.5

Online mode: yes

BungeeCord: no

Server/crash log

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Plugin config (optional)

<!-- Replace this with details about your config -->

Details

Description
once inventory sort is spam-clicked, you are able to put a shulker box into the backpack. this plugin is used for inventory sorting.
https://www.spigotmc.org/resources/chestsort-api.59773/

Steps to reproduce
you have a few random items (not more than 4) + a shulker box in your inventory, and spam-click while the backpack is open until the shulker box goes into the opened backpack.

Expected behavior
normally a shulker box can't go into a backpack, as it essentially works as a shulker box. it even would give me an error message (which disappears when i close my inventory) along the lines of, i assume, "shulker box cannot go into backpack"

Other information (e.g. detailed explanation, related issues, suggestions how to fix, links for us to have context, screenshots, etc.)