Return CascadingDropDown SelectedValue From ModalPopupExtender

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C#

ASP.NET

Ajax

I have a textbox inside a gridview's empty data tag. On textbox OnClick a ModalPopupExtender is activated which displays two (2) CascadingDropDown controls.

When a value is selected in CascadingDropDown1 then CascadingDropDown2 is populated. When I click the "OK" button I want the SelectedValue from CascadingDropDown2 to be populated into TextBox_OrderLineItemPart_InsertFirst.Text

I have an onclick method for Button_Ok but it won't fire and all it does is give me the following error: Microsoft JScript runtime error: 'Button_Ok_Click' is undefined

I have googled the hell out of this for half a day with no resolution.
Thank-you for any assistance you can offer.

<EmptyDataTemplate>
    <asp:TextBox runat="server" 
        ID="TextBox_OrderLineItemPart_InsertFirst" 
        ReadOnly="False"/>
    <asp:ModalPopupExtender runat="server"
        ID="ModalPopupExtender_TextBox_OrderLineItemPart_InsertFirst"
        BackgroundCssClass="modalBackground"
        PopupControlID="Panel_PartModal"
        TargetControlID="TextBox_OrderLineItemPart_InsertFirst" 
        OkControlID="Button_Ok" 
        CancelControlID="Button_Cancel" />

    <asp:Panel runat="server" 
        ID="Panel_PartModal" 
        CssClass="modalPopup" 
        style="display:none;">  
        <table>
            <tr>
                <td>
                <asp:Label runat="server"
                    ID="Label11"
                    Text="Vendor:"/>
                </td>
                <td>
                    <asp:DropDownList runat="server"
                        ID="DropDownList_Vendor" />
                </td>
            </tr>
            <tr>
                <td>
                    <asp:Label runat="server"
                        ID="Label10" 
                        Text="Part:" />
                </td>
                <td>
                    <asp:DropDownList runat="server"
                        ID="DropDownList_Part" />
                </td>
            </tr>
        </table>

    <asp:CascadingDropDown runat="server" 
        ID="CascadingDropDown1" 
        TargetControlID="DropDownList_Vendor" 
        Category="Vendor" 
        PromptText="Select vendor" 
        LoadingText="[Loading vendors ...]"
        ServicePath="~/VendorParts.asmx" 
        ServiceMethod="GetVendorList" />

    <asp:CascadingDropDown runat="server"
        ID="CascadingDropDown2"
        TargetControlID="DropDownList_Part"
        Category="Part" 
        ParentControlID="DropDownList_Vendor"
        PromptText="Select part"
        LoadingText="[Loading vendor parts...]"
        ServicePath="~/VendorParts.asmx"
        ServiceMethod="GetPartsForVendor"/>

        <asp:Button runat="server" 
            ID="Button_Ok" 
            Text="Ok" 
            OnClick="Button_Ok_Click" /> 
        <asp:Button runat="server" 
            ID="Button_Cancel" 
            Text="Cancel" /> 
    </asp:Panel>
</EmptyDataTemplate>

public delegate void PartSelectedHandler(string partNumber);
public event PartSelectedHandler PartNumberSelected;

protected void Button_Ok_Click(object sender, EventArgs e)
{
    GridView GridView_Object = GridView_OrderLineItem;
    DropDownList DropDownList_Part_InsertFirst = ((DropDownList)GridView_Object.Controls[0].Controls[0].Controls[0].FindControl("DropDownList_Part_InsertFirst"));
    TextBox TextBox_OrderLineItemPartNumber_InsertFirst = ((TextBox)GridView_Object.Controls[0].Controls[0].Controls[0].FindControl("TextBox_OrderLineItemPartNumber_InsertFirst"));
    PartNumberSelected(DropDownList_Part_InsertFirst.SelectedValue);
    TextBox_OrderLineItemPartNumber_InsertFirst.Text = PartNumberSelected.ToString();
}