I got this strange error and cant figure how to fix it?

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

public class Loot : MonoBehaviour { public float lootwindowheight = 90; public float Buttonheight = 40; public float Buttonwidth = 40; private List _lootItems; private float OFF_SET = 10; private const int LOOT_WINDOW_ID = 0; private Rect _lootWindow = new Rect(0,0,0,0);

// Use this for initialization
void Start () {
   _lootItems = new List <Item>();

    Populate();
}

// Update is called once per frame
void Update () {

}


 void OnGUI() {
     _lootWindow = GUI.Window(LOOT_WINDOW_ID, new Rect(OFF_SET, Screen.height - (OFF_SET + lootwindowheight), Screen.width - (OFF_SET * 2), lootwindowheight), LootWindow, "Loot Window");
 }

 private void LootWindow(int id) {
     for(int cnt = 0; cnt < _lootItems.Count; cnt++) {
        GUI.Button(new Rect(Buttonwidth * cnt, 0, Buttonwidth, Buttonheight), cnt.ToString());
     }
 }

 private void Populate() {
     for(int cnt = 0; cnt < 25; cnt++)
     _lootItems.Add(new Items());
 }

}