The thrown error is that the element that should be set as FocusedElement cannot be found. Available in LIB version 2016.2.606, it will be also available in the 2016 R2 SP1 release.
As a workaround rather then setting the FocusManager.FocusedElement in the xaml with a binding you can set it in the Loaded event of the RadWindow like this: private void OnRadWindowLoaded(object sender, RoutedEventArgs e) { FocusManager.SetFocusedElement(this, this.textBox); }