DropDownList SelectedIndexChanged event not working when placed inside GridView Footer in ASP.Net

Hello all,

with ref. to this post, http://www.aspsnippets.com/Articles/Bind-DropDownList-in-GridView-FooterTemplate-Footer-Row-in-ASPNet.aspx

I have placed a dropdown list in Gridview footer, now i want to access the dropdown selected indeex changed event as below. But i'm not able to access the event.

 protected void ddlCatg_SelectedIndexChanged(object sender, EventArgs e)
        {
            DropDownList ddlCategories = grvExpenses.FooterRow.FindControl("ddlCatg") as DropDownList;
            DropDownList ddlExpenseSubCategories = grvExpenses.FooterRow.FindControl("ddlSubCatg") as DropDownList;
            ddlExpenseSubCategories.Items.Clear();
            ddlExpenseSubCategories.Items.Insert(0, new ListItem("-Please Select-", "0"));
            int Category = int.Parse(ddlCategories.SelectedValue);
            PopulateExpenseSubCategories(Category);
            
        }
 protected void PopulateExpenseSubCategories(int Category)
        {
            DropDownList ddlExpenseSubCategories = grvExpenses.FooterRow.FindControl("ddlSubCatg") as DropDownList;
            ddlExpenseSubCategories.DataSource = ExpensesBAL.GetExpenseSubCategories(Category);
            ddlExpenseSubCategories.DataTextField = "sub_category";
            ddlExpenseSubCategories.DataValueField = "sub_category";
            ddlExpenseSubCategories.DataBind();
        }

<asp:GridView ID="grvExpenses" runat="server" Width="100%" 
        AutoGenerateColumns="false" EmptyDataText="No Records found."
    AlternatingRowStyle-BackColor="#C2D69B" HeaderStyle-BackColor="green" 
        ShowFooter="true" ondatabound="grvExpenses_DataBound">
    <Columns>
    <asp:TemplateField HeaderText="S.No">
<itemtemplate>
<asp:Label ID="lblSRNO" runat="server" Text='<%#Container.DataItemIndex+1 %>'></asp:Label> 
<asp:Label runat="server" ID="lblId" Text='<%# Bind("s_no") %>' Visible="false"></asp:Label>
</itemtemplate>
</asp:TemplateField>
        <asp:TemplateField HeaderText="Category">
            <ItemTemplate>
                <%# Eval("expense_category")%>
            </ItemTemplate>
            <FooterTemplate>
                <asp:DropDownList runat="server" ID="ddlCatg" AutoPostBack="true">
                <asp:ListItem Text="Please Select" Value="0" Selected="True"></asp:ListItem>
                </asp:DropDownList>
            </FooterTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Sub Category">
            <ItemTemplate>
                <%# Eval("expense_sub_category")%>
            </ItemTemplate>
            <FooterTemplate>
                <asp:DropDownList runat="server" ID="ddlSubCatg" AutoPostBack="true" OnSelectedIndexChanged="ddlCatg_SelectedIndexChanged">
                <asp:ListItem Text="Please Select" Value="0" Selected="True"></asp:ListItem>
                </asp:DropDownList>
            </FooterTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Description">
        <ItemTemplate>
        <%# Eval("Remarks") %>
        </ItemTemplate>
        <FooterTemplate>
       <asp:TextBox ID="txtRemarks" runat="server" />
        </FooterTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Cost">
            <ItemTemplate>
                <%# Eval("cost")%>
            </ItemTemplate>
            <FooterTemplate>
                <asp:TextBox ID="txtcost" runat="server"></asp:TextBox>
            </FooterTemplate>
        </asp:TemplateField>
         <asp:TemplateField HeaderText="Date">
            <ItemTemplate>
                <%# Eval("expense_date")%>
            </ItemTemplate>
            <FooterTemplate>
                <asp:TextBox ID="txtDate" runat="server"></asp:TextBox>
            </FooterTemplate>
        </asp:TemplateField>

            <asp:TemplateField>
            <ItemTemplate>
            </ItemTemplate>
            <FooterTemplate>
                <asp:Button ID="btnAdd" runat="server" Text="Add" OnClick="Add" CommandName = "Footer" />
            </FooterTemplate>
        </asp:TemplateField>
    </Columns>
    <AlternatingRowStyle BackColor="#C2D69B" />
    <EmptyDataTemplate>
        <tr style="background-color: Green;">
            <th scope="col">
                Category
            </th>
            <th scope="col">
                Sub Category
            </th>
            <th scope="col">
            Description
            </th>
            <th scope="col">
                Cost
            </th>
            <th scope="col">
            Date
            </th>
            <th scope="col">
                    
            </th>
        </tr>
          
        <tr>
            <td>
                <asp:DropDownList runat="server" ID="ddlCatg" AutoPostBack="true" OnSelectedIndexChanged="ddlCatg_SelectedIndexChanged">
                <asp:ListItem Text="Please Select" Value="0" Selected="True"></asp:ListItem>
                </asp:DropDownList>
            </td>
            <td>
                <asp:DropDownList runat="server" ID="ddlSubCatg" AutoPostBack="true">
                <asp:ListItem Text="Please Select" Value="0" Selected="True"></asp:ListItem>
                </asp:DropDownList>
            </td>
            <td>
            <asp:TextBox ID="txtRemarks" runat="server" />
            </td>
            <td>
                <asp:TextBox ID="txtcost" runat="server" />
            </td>
            <td>
                <asp:Button ID="btnAdd" runat="server" Text="Add" OnClick="Add" CommandName = "EmptyDataTemplate" />
            </td>
            <td>
            <asp:TextBox ID="txtDate" runat="server"></asp:TextBox>
            </td>
        </tr>
    </EmptyDataTemplate>
</asp:GridView>