When trying to add an item to your Anvil inventory, the following problem occurs:
The item is not displayed to the player. It does not seem to exist, a delay for adding the item is no solution. inv.setItem does not work. Cast Inventory to Anvilinventory does not work.
"This server is running CraftBukkit version git-Spigot-b6ecf3b-9060bfa (MC: 1.12.2) (Implementing API version 1.12.2-R0.1-SNAPSHOT)
You are running the latest version"
Thanks a lot