Error : Missing GUILayout.EndScrollView/EndVertical/EndHorizontal? UnityEditor.DockArea:OnGUI()

Im following the Adventure game tutorial to build an Inventory, in the tutorial they are only using 4 item slots, I need a few more than that. But when I try to add even one more slot I get the following errors when i try to click on the editor down arrow (on the far right) "for item slot 4":

All I have done is change the number of slots from 4 to 5:

 using UnityEngine;
 using UnityEngine.UI;
 using System.Collections;
 
 public class Inventory : MonoBehaviour
 {
     public Image[] itemImages = new Image[numItemSlots];
     public Item[] items = new Item[numItemSlots];
     public const int numItemSlots = 5;
 
 
 
     public void AddItem(Item itemToAdd)
     {
        
         int id = itemToAdd.id;
         if (items.Length <= id)
             return;
         if (items [id]== null)
         {
             items[id] = itemToAdd;
             itemImages[id].sprite = itemToAdd.sprite;
             itemImages[id].enabled = true;
             return;
         }
 
 
        
     }
 
 
 
    public void RemoveItem(Item itemToRemove)
     {
         for (int i = 0; i < items.Length; i++)
         {
             if (items[i] == itemToRemove)
             {
                 items[i] = null;
                 itemImages[i].sprite = null;
                 itemImages[i].enabled = false;
                 return;
             }
         }
     }
 }