Solved Refill Chest

So i created a refill chest method to fill chest in loaded chucks!

But it gave me an error:
Can you guys help me solve this???

CODE:

Code:

    public static ItemStack randomMaterial(){
        chest.clear();

        chest.add(new ItemStack(Material.ARROW, 2));
        chest.add(new ItemStack(Material.GOLDEN_APPLE, 2));
        chest.add(new ItemStack(Material.WOOD_SWORD, 1));
        chest.add(new ItemStack(Material.IRON_HELMET));
        chest.add(new ItemStack(Material.IRON_CHESTPLATE));
        chest.add(new ItemStack(Material.IRON_LEGGINGS));
        chest.add(new ItemStack(Material.IRON_BOOTS));
        chest.add(new ItemStack(Material.CHAINMAIL_HELMET));
        chest.add(new ItemStack(Material.CHAINMAIL_CHESTPLATE));
        chest.add(new ItemStack(Material.CHAINMAIL_LEGGINGS));
        chest.add(new ItemStack(Material.CHAINMAIL_BOOTS));
        Random r = new Random();
        ItemStack rand = new ItemStack(chest.get(r.nextInt()));
        return rand;
    }

    public static void fill(Inventory inv){
        inv.clear();
        for(int i = 0; i <= 26; i++){
            Random r = new Random();

            if(i == r.nextInt(26)){
                inv.setItem(i, randomMaterial());
            }
        }
    }

    public static void blocks(){
        for (World w : Bukkit.getWorlds()){
            for(Chunk c : w.getLoadedChunks()){
                for(BlockState b : c.getTileEntities()){
                    if(b instanceof Chest){
                        Inventory inv = ((Chest) b).getBlockInventory();
                        fill(inv);
                    }
                }
            }
        }
    }

Error:
https://gyazo.com/315486b406fb84571d0123dd2749b2a6


FIXEX: i forgot to add chest.size()