If you open a custom inventory with InventoryType as SHULKER_BOX, add an item to the inventory and then close the inventory, the JVM will crash.

This issue does not occur if you close the inventory without putting something inside (open the inventory then close it without doing anything).

Test code:

@Override
public boolean onCommand(CommandSender sender, Command command, String label, String[] args) {
		Player player = (Player) sender;
		Inventory inv = Bukkit.createInventory(player, InventoryType.SHULKER_BOX);
		player.openInventory(inv);
		return true;
	}