DNN 4.5.3 ClientAPI Question

I am attempting to pass the selected value from a dropdownlist to a client side function but my code throws a 'Object reference not set to an instance of an object' exception when I try to register the client side variable. Please take a look and see if you can help.

In the .ascx file I have

<scripttype="text/javascript"src="/js/des.js">script> 

<asp:DropDownListID="DDL_Report_List"runat="server"AutoPostBack="True"OnSelectedIndexChanged="DDL_Report_List_SelectedIndexChanged" />

In the ascx.cs file in the page load I have:

 private string strURL;        //global var

 protectedvoid Page_Load(object sender, EventArgs e){protectedvoid DDL_Report_List_SelectedIndexChanged(object sender, EventArgs e)  

{

  if (!IsPostBack)    

   {ReportingService2005 rs = InstantiateRS();  

     Get_Report_List(rs);

     Bind_DDL_Report_List(DDL_Report_List); }

     else  {

        if (DDL_Report_List.SelectedIndex != 0) { 

        strURL = DDL_Report_List.SelectedValue;

        if(ClientAPI.BrowserSupportsFunctionality(ClientAPI.ClientFunctionality.DHTML))

           ClientAPI.RegisterClientReference(this.Page, ClientAPI.ClientNamespaceReferences.dnn); 

          try     {ClientAPI.RegisterClientVariable(this.Page, "rURL", strURL, true);}   //THIS IS THE LINE THAT THROWS THE EXCEPTION

          catch (Exception ex){ string temp = ex.ToString();}

    DDL_Report_List.SelectedIndex = 0;

  }

}

And then 

protected void DDL_Report_List_SelectedIndexChanged(object sender, EventArgs e){

string getReport = DDL_Report_List.SelectedValue; 

if(DDL_Report_List.SelectedIndex==2) //set global variable

strURL = @"http://10.0.3.40/Reports/Pages/Report.aspx?ItemPath=/BrokerReports/" + getReport;}

else //set global variable

strURL = @"http://10.0.3.40/Reports/Pages/Report.aspx?ItemPath=/BrokerReports/" + getReport + "?ReportID=CPC1&Firm=E&&rc:parameters=False&rc:Toolbar=false"; 

}

 In the js file I have:

Function GetRpt()

{

var url = dnn.getVar('rURL');

window.open(url, "one", "left=10, top=10, width=600, height=800, location=no, menubar=no, resizable=no, scrollbars=no, status=no, titlebar=no, toolbar=no");

}

I have read the ClientAPI help doc and thik I may have to use ClientAPI.RegisterPostBackEvent, but can't quite grasp everything I am to do. Any help would be greatly appreciated. I know I still have to figure ut how to call GetRpt in the js file. Thanks in advance

Rick Strahl takes this concept further in this article:

www.west-wind.com/weblog/posts/252178.aspx

Thanks, I'll take a look and consider your suggestion. I agree, it is a pain to use the ClientAPI, but initiallly I attempted coding and using client script the conventional way and did not have success. I'll revisit the issue.