DiagramToolBox ItemsSource is set runtime. Selected Index is set to 0. This displays no items. Items are displayed on the next user selection in the ToolBox.
The workaround is to surrond the setting of SelectedIndex with Dispatcher.

   Dispatcher.BeginInvoke(new Action(()=>
            this.tBox.SelectedIndex = 0), DispatcherPriority.Loaded);



Available in R3 2016 Release