Unfortunately, creating a new Web Application using the default templates with Visual Studio 2012 RC and dragging some RadControls from the toolbox to your page leads to this exception. You could read more about it in this blog post
, where you could find some workarounds for the issue.
Another workaround which is not listed in the blog post is to create a new Telerik RadControls Web Site/Web application through Telerik menu in Visual Studio. Dragging controls to your page should not lead to any exceptions and errors.
I hope this information is helpful. Let me know if you have any further questions.
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed