[FIXED] Nullpointer exception when you did not set the display item propperly.

When you are not holding an item while adding a new perk you will now get an error message (please delete your old lang.yml file to see the propper message). All items in the shop that are somehow null or air will be set to a invalid block so you can remove them with the '/pshop remove' command.

Thank you @cyklonix for noticing this bug and reporting it in the discussion tab.