CloseInventory

public class CloseInventory implements Listener {

    @EventHandler
    public void onInventoryClose(InventoryCloseEvent event) {

        ItemStack item = event.getPlayer().getEquipment().getItemInMainHand();
        System.out.println("event?");

        if (item.getItemMeta().getPersistentDataContainer().has(new NamespacedKey(Backpack.getPlugin(), "backpack_id"), PersistentDataType.STRING)){

            System.out.println("true");
            HashMap<Integer, ItemStack> prunedContents = new HashMap<>();

            @Nullable ItemStack @NotNull [] content = event.getInventory().getContents();


            for (int i = 0; i < content.length; i++){
                if (content[i] != null){

                    prunedContents.put(i, content[i]);

                }
            }

//            Arrays.stream(event.getInventory().all(!Material.AIR))
//                    .filter(Objects::nonNull)
//                    .forEach((ItemStack) -> prunedContents.put(ItemStack, ItemStack));

            BackpackUtils.storeItems(prunedContents, item);

        }
    }
}