Since you are using RadControls for ASP.NET AJAX, you should take into
consideration that in MS AJAX environment, all Ajax controls (including Telerik) are loaded after the page itself is loaded (e.g. after
window.onload) in Sys.Application.Load(). That means, you need to make sure that the controls are loaded on the page before trying to use them.
Another option is opening the window from server code as shown below:
sender, EventArgs e)
RadWindow newWindow =
Also check the following links: