ItemStack[] inventory = player.inventory.mainInventory;
for(int i = 0; i < inventory.length; i++) {
if(inventory[i] != null && inventory[i].getItem() instanceof ItemBackpackBase) {
counter++;
if(counter > ConfigurationBackpack.MAX_BACKPACK_AMOUNT) {
player.entityDropItem(inventory[i].copy(), 0);
inventory[i] = null;
}
}
}