I tested here with the version you listed but when I added Telerik.Windows.Data.dll
everything worked fine and I did not get this error. I advise you to check if Telerik.Windows.Data.dll
assembly is not red in the references(failed to load for some reason) and also check if you have added Telerik.Windows.Controls
assembly as well.
I have attached the project I used with the .dlls I used. Please download it and see if it is working on your machine and maybe check for differences between my project and yours.
Please let me know if you need more help. If the issue is still there, could you please send a project with the .dlls you are using included so that I can investigate it further. You zip the project files and attach them to a new support thread.
the Telerik team