I am not quite sure what my cause the problem in your case. Could you please rework the attached demo, open a new support ticket and send it back? I will check it and do my best to provide a working solution.
Please note that I used the OnClientClose event, instead of the ClientCallBackFunction function and I recommend you do the same in your project.The callback function is obsolete and we will remove it from the RadWindow's API in one of the future releases.