Unable to find style 'Close Window Button' in skin 'ChestSkin' Repaint

Hi ! when I open my chest in game i had this error with GUI :

 Unable to find style 'Close Window Button' in skin 'ChestSkin' Repaint

Here is all code for chest:

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 public class MyGUI : MonoBehaviour {
     public GUISkin MySkin;
     
     public float lootWindowHeight = 90;
     
     public float buttonWidth = 40;
     public float buttonHeight = 40;
     public float closeButtonWidth = 20;
     public float closeButtonHeight= 20;
     
     
     private bool _displayLootWindow = false;
     private float _offset = 10;
     private const int LOOT_WINDOW_ID = 0;
     private Rect _lootWindowRect = new Rect(0,0,0,0);
     private Vector2 _lootWindowSlider = Vector2.zero;
     public static Chest chest;
 
     // Use this for initialization
     void Start () {
     }
     
     private void OnEnable() {
         Messenger.AddListener("DisplayLoot", DisplayLoot);
         Messenger.AddListener("CloseChest", ClearWindow);
     }
     
     private void OnDisable() {
         Messenger.RemoveListener("DisplayLoot", DisplayLoot);
         Messenger.RemoveListener("CloseChest", ClearWindow);
     }
     
     // Update is called once per frame
     void Update () {
     
     }
     
     void OnGUI() {
         GUI.skin = MySkin;
         
         if(_displayLootWindow)
             _lootWindowRect = GUI.Window(LOOT_WINDOW_ID, new Rect(_offset, Screen.height - (_offset + lootWindowHeight), Screen.width - (_offset * 2), lootWindowHeight), LootWindow, "Loot Window", "box"); 
     }
     
     private void LootWindow(int id) {
         GUI.skin = MySkin;
         
         if(GUI.Button(new Rect(_lootWindowRect.width - _offset * 2, 0, closeButtonWidth, closeButtonHeight), "x", "Close Window Button"))        
             ClearWindow();
         
         if(chest == null)
             return;
         
         _lootWindowSlider = GUI.BeginScrollView(new Rect(_offset * .5f, 15, _lootWindowRect.width - _offset, 70), _lootWindowSlider, new Rect(0, 0, (chest.loot.Count * buttonWidth) + _offset, buttonHeight + _offset));
         
         for(int cnt = 0; cnt < chest.loot.Count; cnt++) {
             if(GUI.Button(new Rect(_offset * .5f + (buttonWidth * cnt), _offset, buttonWidth, buttonHeight), chest.loot[cnt].Name)) {
                 chest.loot.RemoveAt(cnt);
             }            
         }
         
         GUI.EndScrollView();
     }
     
     private void DisplayLoot() {
         _displayLootWindow = true;
     }
     
     private void ClearWindow() {
         _displayLootWindow = false;
         
         chest.OnMouseUp();
         
         chest = null;
     }
 }