To reproduce:
- Implement the example from this page http://www.telerik.com/help/winforms/pageview-how-to-editing-page-tabs.html.
- Set the page text to empty string and click on another page.


Workaround:
subscribe to the editor's element RadPropertyChanging event:
void element_RadPropertyChanging(object sender, RadPropertyChangingEventArgs args)
{
    if (args.Property == RadElement.ContainsFocusProperty)
    {
        if (!(bool)args.NewValue)
        {
            args.Cancel = true;
        }
    }
}