RenderControl causes The ScriptManager must appear before any controls that need it.

I'm loading a user control in a service handler (.ashx).  In the user control I have a few collapseable panel extenders, a filter text box extender and a modal popup extender.
We're using a master page that has a ScriptManager defined in the layout.
Here's some code:
public void ProcessRequest(HttpContext context)
{
   HtmlTextWriter writer = new HtmlTextWriter(new System.IO.StringWriter());
   MyNamespace.Web.PageBase pageBase = new MyNameSpace.Web.Base.PageBase();
   pageBase.CancelVerifyRenderingInServerForm = true;
   pageBase.EnableEventValidation = false;
   ItemRatesCtl itemRatesCtl = (ItemRatesCtl)pageBase.LoadControl("/Booking/Rates/ItemRatesCtl.ascx");
   itemRatesCtl.RenderControl(writer);
}

On the RenderControl execution:
The control with ID 'CollapsiblePanelExtenderRentBreakdown' requires a ScriptM ...