Added option to require an empty hand + shift-rightclick instead of a custom item to open the GUI. Please add this to your items.yml if you want to use this feature:
Code (YAML):
# When you set "require-custom-item" to false, you can force players to # have an empty hand to open the GUI when using shift+right-click. # It's recommended to enable this when not requiring a custom item, # because otherwise players cannot place blocks against hoppers anymore. require-empty-hand: false