public void saveNBT(ConfigurationSection section, ItemStack itemStack) {
    if (itemStack.getType() == Material.SKULL_ITEM) {
        ItemStack stack = NbtFactory.getCraftItemStack(itemStack);
        NbtFactory.NbtCompound compound = NbtFactory.fromItemTag(stack);
        String skullOwner = compound.getString("SkullOwner", "test"); //error on this line

        section.set("owner", skullOwner);
    }
}