Essentially, I had your halo hat on in the helm slot, opened up the hat gui (H), tried to put another hat into the hat slot there - it sort of floated beside my head - then took it off and put it back in my inventory. I went to pick it up again, thought better of it - I was going to take off the halo hat - put it back down in the same inventory slot, and my game crashed, giving me this error:

The game crashed whilst updating screen events
Error: java.lang.IllegalArgumentException: Parameter specified as non-null is null: method wiresegal.classy.hats.client.gui.GuiHatInventory.func_184098_a, parameter slotIn

The fool report: https://pastebin.com/3uehyLSq

Happened again. Same error. This time while opening and closing the recipe book from the hat menu.

https://pastebin.com/Mi0fHGqp