Could not load file or assembly 'Telerik.Web.UI, Version=2008.2.723.35

When I add grouping to a radGrid in our application a NullRefrenceException is thrown in grid related code.  The grid is using custom skins and css files and the exception isn't thrown when the custom skins aren't used.  The skins are just adapted versions of the Office 2007 skin and work fine when grouping isn't used.  

Thanks

Object reference not set to an instance of an object.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error:

[NullReferenceException: Object reference not set to an instance of an object.]
   Telerik.Web.UI.GridGroupSplitterColumn.PrepareCell(TableCell cell, GridItem item) +201
   Telerik.Web.UI.GridGroupHeaderDecorator.DecorateItem(GridTableView owner, GridColumn[] columnArray) +233
   Telerik.Web.UI.GridItem.PrepareItemStyle() +261
   Telerik.Web.UI.GridGroupHeaderItem.PrepareItemStyle() +121
   Telerik.Web.UI.RadGrid.PrepareRows(GridTableView view) +132
   Telerik.Web.UI.RadGrid.Render(HtmlTextWriter writer) +92
   System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +27
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) +99
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer) +25
   System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +134
   System.Web.UI.Control.RenderChildren(HtmlTextWriter writer) +19
   System.Web.UI.HtmlControls.HtmlForm.RenderChildren(HtmlTextWriter writer) +163
   System.Web.UI.HtmlControls.HtmlContainerControl.Render(HtmlTextWriter writer) +32
   System.Web.UI.HtmlControls.HtmlForm.Render(HtmlTextWriter output) +51
   System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +27
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) +99
   System.Web.UI.HtmlControls.HtmlForm.RenderControl(HtmlTextWriter writer) +40
   System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +134
   System.Web.UI.Control.RenderChildren(HtmlTextWriter writer) +19
   System.Web.UI.Page.Render(HtmlTextWriter writer) +29
   System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +27
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) +99
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer) +25
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1266

Version Information: Microsoft .NET Framework Version:2.0.50727.3053; ASP.NET Version:2.0.50727.3053

• DataFiledID
• DataFieldParentID

 but fails

here is the sample code

class SiteDataItem
{
private string _text;

private string _url;

private int _id;

private int? _parentId;

public string Text

{

get { return _text; }

set { _text = value; }

}

public string Url

{

get { return _url; }

set { _url = value; }

}

public int ID

{

get { return _id; }

set { _id = value; }

}

public int? ParentID

{

get { return _parentId; }

set { _parentId = value; }

}

public SiteDataItem(int id, int? parentId, string text, string url)

{

_id = id;

_parentId = parentId;

_text = text;

_url = url;

}

}

protected void Page_Load(object sender, EventArgs e)

{

List

<SiteDataItem> siteData = new List<SiteDataItem>();

siteData.Add(

new SiteDataItem(1, null, "All Sites", ""));

siteData.Add(

new SiteDataItem(2, 1, "Search Engines", ""));

siteData.Add(

new SiteDataItem(3, 1, "News Sites", ""));

siteData.Add(

new SiteDataItem(4, 2, "Yahoo", "http://www.yahoo.com"));

siteData.Add(

new SiteDataItem(5, 2, "MSN", "http://www.msn.com"));

siteData.Add(

new SiteDataItem(6, 2, "Google", "http://www.google.com"));

siteData.Add(

new SiteDataItem(7, 3, "CNN", "http://www.cnn.com"));

siteData.Add(

new SiteDataItem(8, 3, "BBC", "http://www.bbc.co.uk"));

siteData.Add(

new SiteDataItem(9, 3, "FOX", "http://www.foxnews.com"));

RadTreeView1.DataTextField =

"Text";

RadTreeView1.DataNavigateUrlField =

"Url";

RadTreeView1.DataFieldID =

"ID";

RadTreeView1.DataFieldParentID =

"ParentID";

RadTreeView1.DataSource = siteData;

RadTreeView1.DataBind();

}

here is the output:

Note:

• I'm using Rad controls for asp.net ajax.
• the sample code copied from the online example.