Although I haven't tested the plugin itself, the code is rather inefficient. Things like FileConfigurations don't need to be reinitialised every time an InventoryClickEvent is fired. This also goes for various other constants.
Overal: Functional, but very inefficient. Although real-world performance is likely fine.
Thank you for your comment ^^
This is my second project. The code may be inefficient as I'm still in the learning phase. I will try to optimize the code as soon as possible.