Add Items to Dropdownlist inside the gridview at edit mode in ASP.Net

hi, i'll like your help to solve an issue. i'm following this link

http://www.aspsnippets.com/Articles/Populate-DropDownList-with-Selected-Value-in-EditItemTemplate-of-GridView-in-ASPNet.aspx

to make my owb DropDown List in my Grid and works fine until this line:

ddlCities.Items.FindByValue(country).Selected = True

here, i've got next error:

Object reference not set to an instance of an object.

but my code is right in affected fields:

Protected Sub RowDataBound(ByVal sender As Object, ByVal e As GridViewRowEventArgs)
        If e.Row.RowType = DataControlRowType.DataRow AndAlso grdLinea.EditIndex = e.Row.RowIndex Then

            Dim ddlCities As DropDownList = DirectCast(e.Row.FindControl("ddlFacturarA"), DropDownList)


            ' Create the command with the sproc name and add the parameter required'
            ddlCities.DataSource = GetData("select UPPER(DSCA_ZONA)as Zona from tb_personal  where dsca_Zona <> 'NULL'group by dsca_zona order by dsca_zona")
            ddlCities.DataTextField = "Zona"
            ddlCities.DataValueField = "Zona"
            ddlCities.DataBind()

            'Add Default Item in the DropDownList
            'ddlCountries.Items.Insert(0, New ListItem("Please select"))

            Dim country As String = Trim(CType(e.Row.FindControl("lblFacturarA"), Label).Text)
            ddlCities.Items.FindByValue(country).Selected = True

        End If
    End Sub

<EditItemTemplate >
                    <asp:label ID="lblFacturarA" Value ='<%# Eval("facturar_a")%>' Visible ="false" runat="server" />
                      
                    <asp:DropDownList 
                        ID="ddlFacturarA"  
                        CssClass="txt"   
                        runat="server" 
                         
                        AutoPostBack="True" ValidationGroup="rfNewLineEmpty">
                    </asp:DropDownList>
                
               

                    <asp:RequiredFieldValidator 
                        ID="rfNewLineFacturarA" 
                        runat="server" 
                        ErrorMessage="Obligatorio" 
                        ValidationGroup="rfNewLine" 
                        SetFocusOnError="True" 
                        ControlToValidate="ddlFacturarA">
                    </asp:RequiredFieldValidator>
    </EditItemTemplate> 

i know i'm new at ASP.NET and maybe i'm loosing something by the way, but i've been round this code for two days and don't see light.

can you tell me something about reason for this error?

thanks in advance