Error Populating Dropdownlist in Gridview Footer Template in ASP.NET

public void GridView1_RowCreated(Object sender, GridViewRowEventArgs e)
    {
        if (e.Row.RowType == DataControlRowType.Footer) {
            DropDownList ddlGrandParent = (DropDownList)e.Row.FindControl("ddlGrandParentService");

                ddlGrandParent.DataSource = GetGrandParentService();
                ddlGrandParent.DataValueField = "GrandParentServiceID";
                ddlGrandParent.DataTextField = "GrandService";
                ddlGrandParent.DataBind();

                //ddlGrandParent.SelectedValue = GridView1.DataKeys[e.Row.RowIndex].Value.ToString();

                DropDownList ddlParent = (DropDownList)e.Row.FindControl("ddlParentService");

          //**Getting error "Object reference not set to an instance of an object."**

           ddlParent.DataSource = GetParentServiceByGrandParent  (Convert.ToInt32((GridView1.FooterRow.FindControl
        ("ddlGrandParentService") as DropDownList).SelectedItem.Value));


                ddlParent.DataValueField = "ParentServiceId";
                ddlParent.DataTextField = "ParentServiceName";
                ddlParent.DataBind();

}
}


private List<GrandParentService> GetGrandParentService()
    {

        List<GrandParentService> all = new List<GrandParentService>();
        using (ABCEntities dc = new ABCEntities())
        {
            all = dc.GrandParentService.ToList();
        }
        return all;
    }


private List<ParentService> GetParentServiceByGrandParent(int grandParentID)
    {

        List<ParentService> all = new List<ParentService>();
        using (ABCEntities dc = new ABCEntities())
        {
            all = dc.ParentService.Where(a => a.GrandParentServiceID.Equals(grandParentID)).ToList();
        }
        return all;

    }