Im getting NullReferenceException error

Im getting NullReferenceException error for this line this.transform.SetParent (inv.slots[slot].transform); in the item data class when trying to move one of the item in the inventory to a another slot.

here is my code public class Inventory : MonoBehaviour {

 GameObject inventoryPanel;
 GameObject slotPanel;
 ItemDatabase database;
 public GameObject inventorySlot;
 public GameObject inventoryItem;

 int slotAount;
 public List<Item> items = new List<Item>();
 public List<GameObject> slots = new List<GameObject>();

 void Start()
 {
     database = GetComponent<ItemDatabase>();

     slotAount = 25;
     inventoryPanel = GameObject.Find("Inventory Panel");
     slotPanel = inventoryPanel.transform.FindChild("Slot Panel").gameObject;

     for (int i = 0; i < slotAount; i++)
     {
         items.Add(new Item());
         slots.Add(Instantiate(inventorySlot));
         slots [i].GetComponent<Slot> ().id = i;
         slots[i].transform.SetParent(slotPanel.transform);
     }
     AddItem(2);
     AddItem(1);
 }

 public void AddItem(int id)
 {

     Item itemToAdd = database.FetchItemByID(id);
     if (itemToAdd.Stackable && CheckItem(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> ().slot = i;
                 itemObj.transform.SetParent(slots[i].transform);
                 itemObj.transform.position = Vector2.zero;
                 itemObj.GetComponent<Image>().sprite = itemToAdd.Sprite;
                 itemObj.name = itemToAdd.Title;
                 break;
             }
         }
     }
 }

 bool CheckItem(Item item)
 {
     for (int i = 0; i < items.Count; i++)

         if (items[i].ID == item.ID)

             return true;


     return false;
 }

}

public class ItemData : MonoBehaviour, IBeginDragHandler, IDragHandler, IEndDragHandler { public Item item; public int amount; public int slot;

 private Inventory inv;
 private Vector2 offset;

 void Strat()
 {
     inv = GameObject.Find ("Inventory").GetComponent<Inventory> ();    }

 public void OnBeginDrag (PointerEventData eventData)
 {
     if (item != null) 
     {
         offset = eventData.position - new Vector2 (this.transform.position.x, this.transform.position.y);
         this.transform.SetParent (this.transform.parent.parent);
         this.transform.position = eventData.position - offset;
         GetComponent<CanvasGroup> ().blocksRaycasts = false;
     }
 }

 public void OnDrag (PointerEventData eventData)
 {
     if (item != null) 
     {
         this.transform.position = eventData.position - offset;
     }
 }

 public void OnEndDrag (PointerEventData eventData)
 {
     this.transform.SetParent (inv.slots[slot].transform);
     this.transform.position = inv.slots[slot].transform.position;
     GetComponent<CanvasGroup> ().blocksRaycasts = true;


 }

}

public class Slot : MonoBehaviour, IDropHandler { public int id; private Inventory inv;

 void Start()
 {
     inv = GameObject.Find ("Inventory").GetComponent<Inventory> ();
 }

 public void OnDrop (PointerEventData eventData)
 {
     ItemData droppeditem = eventData.pointerDrag.GetComponent<ItemData> ();
     if (inv.items [id].ID == -1) {
         inv.items [droppeditem.slot] = new Item ();
         inv.items [id] = droppeditem.item;
         droppeditem.slot = id;
     } else 
     {
         Transform item = this.transform.GetChild (0);
         item.GetComponent<ItemData> ().slot = droppeditem.slot;
         item.transform.SetParent (inv.slots[droppeditem.slot].transform);
         item.transform.position = inv.slots[droppeditem.slot].transform.position;

         inv.items [droppeditem.slot] = item.GetComponent<ItemData> ().item;
         inv.items [id] = droppeditem.item;

         droppeditem.slot = id;
         droppeditem.transform.SetParent (this.transform);
         droppeditem.transform.position = this.transform.position;


     }
 }

}