Thanks for using our controls. Unfortunately, we were not able to reproduce the described error I suggest that you try to remove and then add the references to the Telerik assemblies again. To use the RadDocking control you have to add reference to Telerik.Windwos.Controls, Telerik.Windwos.Controls.Navigation Telerik.Windows.Controls.Docking assemblies. Please let us know if you need more help.
All the best,
the Telerik team