Bug Fixes: In this update, I changed the way the GUI click listener works. It will now work safer and better for your server! Many bugs with lower versions using the GUI were also fixed.
Please note that if you are using the GUI with spigot version 1.12 or lower then you cannot use ID numbers! All materials must be given in its name. This means you will not be able to use many of the blocks in Minecraft that change color. I do not plan on fixing this, if you want the ability to use all the blocks possible in the GUI please update your server to 1.13+.