Error with chests

Okay so a code im using is this:

Code:

private HashMap<UUID, Inventory> backpacks = new HashMap<UUID, Inventory>();
   
    @EventHandler
    public void onPlayerJoin(PlayerJoinEvent e) {
        Inventory inv = Bukkit.getServer().createInventory(e.getPlayer(), InventoryType.CHEST);
       
        if (getConfig().contains("backpacks." + e.getPlayer().getUniqueId())) {
            for (String item : getConfig().getConfigurationSection("backpacks." + e.getPlayer().getUniqueId()).getKeys(false)) {
                inv.addItem(loadItem(getConfig().getConfigurationSection("backpacks." + e.getPlayer().getUniqueId() + "." + item)));
            }
        }
       
        backpacks.put(e.getPlayer().getUniqueId(), inv);
    }
   
    @EventHandler
    public void onPlayerLeave(PlayerQuitEvent e) {
        if (!getConfig().contains("backpacks." + e.getPlayer().getUniqueId())) {
            getConfig().createSection("backpacks." + e.getPlayer().getUniqueId());
        }
       
        char c = 'a';
        for (ItemStack itemStack : backpacks.get(e.getPlayer().getUniqueId())) {
            if (itemStack != null) {
                saveItem(getConfig().createSection("backpacks." + e.getPlayer().getUniqueId() + "." + c++), itemStack);
            }
        }
       
        saveConfig();
    }
   
    public void onEnable() {
        Bukkit.getServer().getPluginManager().registerEvents(this, this);
        getConfig().options().copyDefaults(true);
        saveConfig();
        saveDefaultConfig();
    }
   
    public void onDisable() {
        for (Entry<UUID, Inventory> entry : backpacks.entrySet()) {
            if (!getConfig().contains("backpacks." + entry.getKey())) {
                getConfig().createSection("backpacks." + entry.getKey());
            }
           
            char c = 'a';
            for (ItemStack itemStack : entry.getValue()) {
                if (itemStack != null) {
                    saveItem(getConfig().createSection("backpacks." + entry.getKey() + "." + c++), itemStack);
                }
            }
           
            saveConfig();
        }
    }
   
    @Override
    public boolean onCommand(CommandSender sender, Command cmd, String commandLabel, String[] args) {
        if (!(sender instanceof Player)) {
            sender.sendMessage(ChatColor.RED + "The console cannot have an backpack.");
            return true;
        }
       
        Player p = (Player) sender;
       
        if (cmd.getName().equalsIgnoreCase("backpack")) {
            p.openInventory(backpacks.get(p.getUniqueId()));
        }
       
        return true;
    }
   
    private void saveItem(ConfigurationSection section, ItemStack itemStack) {
        section.set("type", itemStack.getType().name());
        section.set("amount", itemStack.getAmount());
        // Save more information.
    }
   
    private ItemStack loadItem(ConfigurationSection section) {
        return new ItemStack(Material.valueOf(section.getString("type")), section.getInt("amount"));
        // Load more information.
    }

But when i do /backpack it gives an error on command:

Code:

p.openInventory(backpacks.get(p.getUniqueId()));

 

Can we have the error log?