Remove the DataTextField line and the problem will be resolved.
The problem in this scenario is that the DataTextField instructs the text method that the text is stored in the "Text" field. This can be true if the tree was loaded via AJAX; however, the TreeView is rendered from the server, so the datasource uses the default, lowercase-t "text" field. So, the text method returns undefined.
Working sample is attached.
Check out the Telerik Platform - the only platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native mobile apps.