When using the Player.openInventory(Inventory) method, you are not able to open the inventories manually created with Server.createInventory(InventoryHolder, InventoryType) with the following inventory types: CARTOGRAPHY, CRAFTING, CREATIVE, GRINDSTONE, LECTERN, LOOM, MERCHANT, The openinventorylogs.txt file shows the console output of the exception that is generated.

To replicate this, upload the BugTestingPlugin.jar plugin to your server and run the command /openInventory with one of the inventory types listed above as the first argument.

Ex. /openInventory LOOM

This will generate the internal error associated with this exception. To verify that the command works properly, run "/openInventory ANVIL" to show that it does in fact open other inventory types.

Here's some of the example code that has been causing this issue:

InventoryType it = InventoryType.valueOf(args[0]);
if (it == null) {
sender.sendMessage("Please specify a valid inventory type.");
return true; 
}
Player p = (Player) sender;  Inventory i = Bukkit.getServer().createInventory(null, it);
p.openInventory(i);

(I did a check prior to make sure the sender was a player.)

While I might understand not being able to open a crafting or creative inventory through this method, all of the other inventories listed should be accessible without generating this exception.