NRE with my plugin, unsure how to debug

I searched in forum but nothing help me solving this.

Well, i dont have much to say. This is my code:

Code:

Failed to call hook 'OnItemCraft' on plugin 'TestSkin v1.0.0' (NullReferenceException: Object reference not set to an instance of an object)

Code:

using System.Linq;namespace Oxide.Plugins
{
    [Info("TestSkin", "testing", 1.0)]
    [Description("teeeeeeest")]
    class TestSkin : RustPlugin
    {
        private void Loaded()
        {
            LoadDefaultConfig();
            permission.RegisterPermission("test.testing", this);
        }
           
            private void OnItemCraft(ItemCraftTask task, BasePlayer crafter, BasePlayer player)
            {
                 if (!HasPermission(player, "test.testing")) //I THINK THIS IS THE PROBLEM
                {
                    SendReply(player, "Normal Cloth");
                    return;
                }
                var skins = ItemSkinDirectory.ForItem(task.blueprint.targetItem);
                if (skins.Count() < 2) return;
                task.skinID = skins.GetRandom().id;
            }
 
            private void OnItemCraftFinished(ItemCraftTask task, Item item)
            {
                if (task.amount == 0) return;
                var skins = ItemSkinDirectory.ForItem(task.blueprint.targetItem);
                if (skins.Count() < 2) return;
                task.skinID = skins.GetRandom().id;
            }
    bool HasPermission(BasePlayer player, string perm) => permission.UserHasPermission(player.UserIDString, perm);
    }
}

Thank you!!!