Object reference not set to an instance of an object error

"Object reference not set to an instance of an object" 

 protected void gvShowRequests_RowDataBound(object sender, GridViewRowEventArgs e)
        {
            DropDownList gvddlRequestStatus = (DropDownList)e.Row.FindControl("gvddlRequestStatus");
            this.PopulateRequestStatus(gvddlRequestStatus);
        }


private void PopulateRequestStatus(DropDownList ddl)
        {
            DataTable requestStatus = (new RequestStore()).GetReqStatusList();
            
            ddl.DataSource = requestStatus;
            ddl.DataTextField = "Status";
            ddl.DataValueField = "Id";
            ddl.DataBind();

            if (ddl.Items.Count > 1)
            {
                ddl.Items.Insert(0, new ListItem(ApplicationConstants.dropdownDefaultValue, "0"));
                ddl.Items[0].Value = "0";
            }

        }

<asp:GridView ID="gvShowRequests" runat="server" AllowPaging="true" PageSize="15" AutoGenerateColumns="false" Width="60%" AllowSorting="false" GridLines="Both" BorderColor="Black" PagerStyle-CssClass="pager" OnPageIndexChanging="gvShowRequests_PageIndexChanging" BorderStyle="Groove" OnRowDataBound="gvShowRequests_RowDataBound">
    <RowStyle CssClass="gvRows" />
    <AlternatingRowStyle CssClass="gvRowsalt" />
    <Columns>
    <asp:BoundField DataField="Id" HeaderText="Id" Visible="false" />
    <asp:BoundField DataField="Name" HeaderText="Application" />
    <asp:BoundField DataField="CostCentre" HeaderText="Cost Centre" />
    <asp:BoundField DataField="UserName" HeaderText="User Name" />
    <asp:BoundField DataField="Type" HeaderText="Request Type" />
    <asp:TemplateField HeaderText="Request Status--" SortExpression="Status">
     <ItemTemplate>
      <asp:DropDownList ID="gvddlRequestStatus" runat="server">
      </asp:DropDownList>
      </ItemTemplate>
      </asp:TemplateField>
      <asp:BoundField DataField="Status" HeaderText="Request Status" />
      <asp:BoundField DataField="SentBy" HeaderText="Sent By" />
      <asp:BoundField DataField="AddedOn" HeaderText="Added On" />
      </Columns>
      </asp:GridView>