PrepareItemCraftEvent Bug

    @EventHandler
    public void onPrepareCraftItem(PrepareItemCraftEvent event)
    {
        if(event.getRecipe() == null)
        {
            Player player = (Player) event.getView().getPlayer();
            CraftingInventory craftingInventory = event.getInventory();
            ItemStack[] matrix = craftingInventory.getMatrix();
            if(matrix[0] != null && matrix[1] != null)
            {
                if(matrix[0].getType().equals(Material.DIRT) && matrix[1].getType().equals(Material.COBBLESTONE))
                {
                    for(int i = 2; i < matrix.length; i++)
                    {
                        if(matrix[i] != null)
                        {
                            return;
                        }
                    }
                    event.getInventory().setResult(new ItemStack(Material.GRAVEL));
                }
            }
        }
    }