Adding a non-monobehaviour script to the execution order

So I'm trying to create an inventory system based on a tutorial, and at some point(specifically when I was trying to add to a list the "items", with icons and everyhting) I get an out of range error.The tutorial specifies that in order to remove that error I have to set a specific execution order for the 3 scripts I'm currently using.

The problem is, one of the scripts is not a monobehaviour , and while he simply adds it to the order, I can't seem to do it, and I don't see another way of removing the error.

using UnityEngine;

using System.Collections;

[System.Serializable]

 public class Item {
 public string itemName;
 public int itemID;
 public string itemDesc;
 public Texture2D itemIcon;
 public int itemPower;
 public int itemSpeed;
 public ItemType itemType;

 public enum ItemType {
     Weapon,
     Consumable,
     Quest
 }

 public Item (string name, int id, string desc, int power, int speed, ItemType type)
 {
     itemName = name;
     itemID = id;
     itemDesc = desc;
     itemIcon = Resources.Load<Texture2D> ("Item Icons/"  + name);
     itemPower = power;
     itemSpeed = speed;
     itemType = type;
 }

}

I don't know if they're necesary, but here are the other 2 scripts

    using UnityEngine;
     
     using System.Collections;
     
     using System.Collections.Generic;
     
     public class ItemDatabase : MonoBehaviour {
         public List<Item> items = new List<Item> ();
     
         void Start ()
         {
             items.Add (new Item ("Rotten Apple",0,"Things can be seen moving under the surface of this putrid apple",2,0, Item.ItemType.Weapon));
         }
     }

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

 public class Inventory : MonoBehaviour {
     public List<Item> inventory = new List<Item> ();
     public ItemDatabase database;

 // Use this for initialization
 void Start () {
     database = GameObject.FindGameObjectWithTag ("Item Database").GetComponent<ItemDatabase>();
     inventory.Add (database.items[0]);

         
 }
 
 void OnGUI ()
 {
     for (int i = 0; i < inventory.Count; i++)
     {
         GUI.Label (new Rect (10,i * 20, 200, 50), inventory[i].itemName);
     }
 }


 }