I tested the project and was able to replicate the error. I assume that the cause of it is the OnZoomEnd event firing too early, before the client-side objects of the controls, i.e. the RadAjaxManager, are created. However I will need a bit more time to perform further investigation on it.
In the meanwhile could you please confirm that the map control supports ajax? You can also try ajaxifying it with ASP:UpdatePanel and see if it works thus.
All the best,
the Telerik team