In my plugin I simply check the item material to see if the player is auctioning air and I disallow them to do so, however all the new items I've tested so far seem to show up as Material.AIR when I check. Possible I'm doing something wrong however this method works on all 1.12 and below items.

Code: https://pastebin.com/FEiZaJDp

Console Output: https://pastebin.com/FTY7GhC0