GUI crashes when shift left clicking on empty slot

I've created a GUI, shift left clicking on items works fine, and plain left clicking works too, but whenever I shift left click on an empty slot the game crashes (shutting internal server, game closes). I tried creating a new workspace, I created a block with inventory, a default GUI (only with inherited from block inventory) and a procedure to open the GUI right clicking the block. Still, no luck, it kept crashing. 

I've sent the workspace, the crash report and mcreator's console log. And if it's not a bug I'd really like to know how to solve it.