How to assign the control paramater to sqldatasource in code behind page.

I ran into an error "object data not set refererece " ( most dreaded error message) at the point where the control parameter was assigned to the sqldatasource in the aspx page.

<asp:SqlDataSource ID="SqlDataSourceAppliedDateScreenDateCheck" runat="server"
ConnectionString="<%$ ConnectionStrings:ConnectionString1%>"
SelectCommand="PROC1"
SelectCommandType="StoredProcedure" DataSourceMode="DataReader"><SelectParameters><asp:ControlParameter ControlID="DropDownListCode" Name="NewCode"
PropertyName="SelectedValue" Type="Int32" /></SelectParameters></asp:SqlDataSource>
The DropdowListCode was not accessible because it is inside a FormView.I changed the above as follows to make it work.

Page1.aspx <asp:SqlDataSource ID="SqlDataSourceAppliedDateScreenDateCheck" runat="server" ConnectionString="<%$ ConnectionStrings:ConnectionString1%>"
SelectCommand="PROC1"
SelectCommandType="StoredProcedure" DataSourceMode="DataReader">
</asp:SqlDataSource>

// removed the control parameter to add in the code behind page
Page1.aspx.vb

Protected Sub SqlDataSourceAppliedDateScreenDateCheck_Selecting(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.SqlDataSourceSelectingEventArgs) Handles SqlDataSourceAppliedDateScreenDateCheck.Selecting
'Pass the control parameter in the code behind

Dim drpNewCode As DropDownList = DirectCast(FormView1.FindControl("DropDownListCode"), DropDownList)
Dim sqlCtrlParam As SqlParameter = New SqlParameter("@NewCode", drpNewCode.SelectedValue.ToString())
e.Command.Parameters.Add(sqlCtrlParam)
End Sub