Calling 'AddItem' Method giving me error NullReferenceException:

So i have a json. Data base that has id's set to each object. the Method 'AddItem(0)' is suppose to add a Weak health Potion to the UI inventory but gives an error. i have correctly initialized the inventory script using inv = GetComponent<Inventory>(); in the the start and have a variable up top as public Inventory inv;

This is the Keyboard input Script in the Update:

 if (Input.GetKeyUp(KeyCode.E))
         {
             if (ray.hit.transform.tag == "Weak Health Potion")
             {
                // Gives error Here
                 inv.AddItem(0);
                 //------------------
                 print("Picked up");
                 Destroy(ray.hit.transform.gameObject);
                 
             }
         }

And This is the Inventory Script piece:

       public void AddItem(int id)
     {
 
         Item itemToAdd = database.FetchItemByID (id);
 
         if (itemToAdd.Stackable && CheckIfItemIsInInventory (itemToAdd)) {
 
             for (int i = 0; i < items.Count; i++) {
                 if (items [i].ID == id) {
                     ItemData data = slots [i].transform.GetChild(0).GetComponent<ItemData> ();
                     data.amount ++;
                     data.transform.GetChild(0).GetComponent<Text> ().text = data.amount.ToString ();
                     break;
                 }
             }
         }
         else
         {
             for (int i = 0; i < items.Count; i ++) {
 
                 if (items [i].ID == -1) {
                     items [i] = itemToAdd;
                     GameObject itemObj = Instantiate (inventoryItem);
                     itemObj.GetComponent<ItemData>().item = itemToAdd;
                     itemObj.GetComponent<ItemData>().amount = 1;
                     itemObj.GetComponent<ItemData>().slot = i;
                     itemObj.transform.SetParent (slots [i].transform);
                     itemObj.transform.position = Vector2.zero;
                     itemObj.GetComponent<Image> ().sprite = itemToAdd.Sprite;
                     itemObj.name = itemToAdd.Title;
                     break;
                 }
 
             }
 
         }
     }

` Im not sure why it is giving error: Please Help me. I am sure its something my tried brain missed.

 NullReferenceException: Object reference not set to an instance of an object
 KeyboardInput.Update () (at Assets/Scripts/KeyboardInput.cs:50)