Refresh Workbench Inventory?

I am working on making some custom item recipes for my server, I got everything working, but the item in slot 0 (the "finished product" when crafting) is invisible and there is no updateInventory() method for workbenches.

Code:

    @EventHandler
    public void crafting(InventoryClickEvent event) {
        System.out.println(event.getSlot());
        if (event.getInventory().getType() == InventoryType.WORKBENCH) {
            if (!event.getInventory().getItem(3).equals(null) && event.getInventory().getItem(3).equals(r.item1) && !event.getInventory().getItem(5).equals(null) && event.getInventory().getItem(5).equals(r.item2)) {
                System.out.println("Confirmed");
                event.getInventory().setItem(0, f);   //THIS item is the "finished product", but it is invisible
            }
        }
    }

Before you ask why I don't just make a ShapedRecipe is because my items are custom named, and recipes require a Material, not ItemStack....

 

player.updateInventory()

 

already tried

it's not a players inventory, so it would not work

 

I tried setting slot 0 to an item, then adding a custom meta, but it seems that every item put in slot 0 is invisible.