GUI.Window error. InvalidOperationException: Hashtable.Enumerator: snapshot out of sync.

I wanna to draw a menu. and here is the code, but i come across a problem when i hit Play,and i find the error is caused by this line of code below:GUI.Window(GlobalGUIWindowId._WND2_YXY..... ,and the error in the console window says:

InvalidOperationException: Hashtable.Enumerator: snapshot out of sync. System.Collections.Hashtable+Enumerator.FailFast () System.Collections.Hashtable+Enumerator.MoveNext () UnityEngine.GUI.EndWindows (UnityEngine.IDList idlist) (at C:/BuildAgent/work/f724c1acfee760b6/Runtime/ExportGenerated/Editor/GUI.cs:1225) UnityEngine.GUIUtility.EndGUI (Int32 doLayout, Int32 doWindows, UnityEngine.IDList idlist) (at C:/BuildAgent/work/f724c1acfee760b6/Runtime/ExportGenerated/Editor/GUIUtility.cs:359)

I have no idea how to debug it, may you help me. Thank you very much!

    void OnGUI()
{        
    //
    _rectMainMenu = new Rect(_menuPos.x, _menuPos.y, Screen.width - _menuPos.x, _mainMenuHeight);
    _rectMainMenu = GUI.Window(GlobalGUIWindowId._WND1_YXY, _rectMainMenu, MenuMainWindow, "", _mainMenuBackStyle);
}
void MenuMainWindow(int windowId)
{
    int nNumPopupMenus = _popupMenus.Count;
    float width = 0f;
    float x = 0f;
    for (int i = 0; i < nNumPopupMenus; i++)
    {
        PopupMenu popItem = (PopupMenu)_popupMenus[i];
        popItem.SetStyles(_mainMenuBackStyle, _menuItemStyle, _menuDisableStyle);
        width = GUILayoutUtility.GetRect(new GUIContent(popItem.Text), _popupMenuStyle).width + _spaceBetweenMenuItems;
        print("text, width:" + popItem.Text + ", " + width.ToString());
        Rect rectCur = new Rect(x, 0, width, _popupMenuHeight);
        popItem.DrawPopupMenu(rectCur, _popupMenuStyle);
        x += width;
    }
}



  public void DrawPopupMenu(Rect rect, GUIStyle popupMenuStyle)
{
    GUIContent guiContent = new GUIContent(_text);
    GUI.Label(rect, guiContent, popupMenuStyle);
    //
    //
    _menuItemMaxWidth = 100;//not complete

    rectMax = new Rect(10, 10, _menuItemMaxWidth, 10);
    ***GUI.Window(GlobalGUIWindowId._WND2_YXY, rectMax,  DrawItsMenuItems, "", _mainMenuStyle);//***
}

private void DrawItsMenuItems(int windowId)
{
    //
    //int nNumMenuItems = _menuItems.Count;
    //float y = 0;
    //for (int i = 0; i < nNumMenuItems; i++)
    //{
    //    MenuItem item = (MenuItem)_menuItems[i];
    //    if (item.Show)
    //    {
    //        //
    //        //
    //        _menuItemHeight = 18;
    //        Rect rectMenuItem = new Rect(0, y, _menuItemMaxWidth, _menuItemHeight);
    //        //item.DrawMenuItem(rectMenuItem, _menuItemStyle, _disableMenuItemStyle);
    //    }
    //    y += _menuItemHeight;
    //}
}