Such error could be observed if the parent appointment somehow is not loaded from the datasource because of some filtering of the appointments. Would you please review our Bind To List Demo where is implemented a scenario similar to what you have described and let us know if this error can be reproduced there as well? If not I will recommend you to compare to with your case and do the necessary changes.
Hope this will be helpful.
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed