I suppose that the problem is due to the fact that the id of the hidden field is changed after it is put in another naming container (Panelbar in your case).
You need to find out what is the real client ID of the control, depending on the naming container of the control. The naming containers for a given control is the parent control above it in the master page and TemplateField that implements the INamingContainer Interface.
Please note that if you want to get the control by ID, you need to get the ID of all naming containers not only the control's ID.
I hope this helps.
the Telerik team