Telerik. RadScriptManager issue at RadWindow

i show RadWindow from parent form:

 <telerik:RadTreeView ID="tvFile" runat="server">
        OnClientContextMenuItemClicking="onClientContextMenuItemClicking">
<ContextMenus>
    <telerik:RadTreeViewContextMenu ID="MainContextMenu" runat="server">
        <Items>
             <telerik:RadMenuItem开发者_如何学JAVA Value="Rights" Text="Rights"/>
       </Items>
   </telerik:RadTreeViewContextMenu>
</ContextMenus>
</telerik:RadTreeView>
<telerik:RadWindowManager ID="winManager" ShowContentDuringLoad="false" VisibleStatusbar="false"
    ReloadOnShow="false" runat="server" Skin="Sunset" EnableShadow="true">
    <Windows>
        <telerik:RadWindow ID="RadWindow2" Modal="true" onClientClose="OnClientClose"
        NavigateUrl="UserRights.aspx" runat="server" Behaviors="Close">
        </telerik:RadWindow>
    </Windows>
</telerik:RadWindowManager>

 <script type="text/javascript">
function OnClientClose(oWnd, args)
{
    var arg = args.get_argument();
    if (arg)
     {} 
}
function openWin(path) {
    var oWnd = radopen("UserRights.aspx?file="+path, "RadWindow2");
}


function onClientContextMenuItemClicking(sender, args)
{
    var menuItem = args.get_menuItem();
    var treeNode = args.get_node();
    var nodeValue = treeNode.get_value();
    menuItem.get_menu().hide();
    switch (menuItem.get_value())
    {
        case "Rights":
            openWin(nodeValue);
            break;
    }
}

At UserRights.aspx I need RadScriptManager for RadListBox like this:

 <form id="form1" runat="server">
   <telerik:RadScriptManager ID="RadScriptManager1" runat="server"/>
    <asp:Button runat="server" ID="ds" Text="fd" />
    <telerik:RadListBox runat="server" ID="dcs"></telerik:RadListBox>
 </form>

But when RadWindow opens at my firebug I got js error:

this._form is null Type._registerScript("MicrosoftAjaxWeb...UI._UpdateProgress",Sys.UI.Control);

When I haven't got RadScriptManager all is OK. But I need it for RadListBox and UpdatePanel features.

Does the same problem occur if you show the content page not in a RadWindow, but in an IFRAME? If the problem exists only with RadWindow, set ShowContentDuringLoad=true.