Hello Nesim,
Please make sure that you are using the latest released assemblies from our last internal build.
I tried the example with the following code and it works as expected:
private void Application_Startup(object sender, StartupEventArgs e)
{
this.RootVisual = new Page1();
ChildWindow window = new ChildWindow();
RadHtmlPlaceholder placeHolder = new RadHtmlPlaceholder();
placeHolder.SourceUrl = new Uri("http://www.telerik.com",UriKind.RelativeOrAbsolute);
window.Content = placeHolder;
window.Show();
}
Please let us know if you have any other questions or need some more help or sample code.
Best wishes,
Boryana
the Telerik team