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;
	}
}

Do you have a style called Close Window Button in the GUISkin you use?