Getting error- "Argument is out of range, parameter name: index"

Here is my code....I have a feeling it is to do with my Items1 list not having anything in it, but I don't actually know how to add them to it! Am a noob and got all this from a youtube video.

using UnityEngine; using System.Collections; using System.Collections.Generic; using UnityEngine.UI;

public class CraftField : MonoBehaviour {

 public List<GameObject> Slots1 = new List<GameObject>();
 public List<Item> Items1 = new List<Item>();  //Taken from inventory
 public GameObject slotsTest;
 ItemDatabase database;

 int x = -400;
 int y = 167;


 void Start () {

     database = GameObject.FindGameObjectWithTag("ItemDatabase").GetComponent<ItemDatabase>();

     for(int i = 1; i < 6; i++){
         for(int k = 1; k < 6; k++){
             GameObject slot1 = (GameObject)Instantiate(slotsTest);
             Slots1.Add(slotsTest);
             slot1.transform.parent = this.gameObject.transform;
             slot1.name = "SlotTest" + i + "." + k;
             slot1.GetComponent<RectTransform>().localPosition = new Vector3(x, y, 0f);
             x = x + 200;
             if(k == 5){
                 x = -400;
                 y = y - 90;
             }
         }
     }

 }

 public void checkIfItemExists(int itemID, Item item){
     for (int i = 0; i < Items1.Count; i++){
         if(Items1[i].itemID == itemID){
             Items1[i].itemValue = Items1[i].itemValue + 1;
             break;
         } else if(i == Items1.Count - 1){
             addItemAtEmptySlot(item);
         }
     }
 }
 
 public void addItem(int id){
     for(int i = 0; i < database.items.Count; i++){
         if(database.items[i].itemID == id){
             Item item = database.items[i];
             
             if(database.items[i].itemType == Item.ItemType.Consumable){
                 checkIfItemExists(id, item);
                 break;
             } else{
                 addItemAtEmptySlot(item);
             }
         }
     }
 }
 
 public void addItemAtEmptySlot(Item item){
     for(int i = 0; i < Items1.Count; i++){
         if(Items1[i].itemName == null){
             Items1[i] = item;
             break;    //the loop does not need to continue so we break it
         }
     }
 }

}