Unfortunately we wasn't able to reproduce the described issue on our side.
I made a simple project based on the example in our Demo and it seems to work properly in the scenario described.
Could you please try this project and let us know if you get this exception with it?
If you encounter this exception only in your project, it will be best if you can send it to us, in order to investigate the problem. It will also be helpful for us to know the exact version of the controls you are using.
Thank you in advance.
the Telerik team