DropDownList FindByValue: System.NullReferenceException: Object reference not set to an

Error:ddlfilter.Items.FindByValue(strfilter).Selected = true;

protected void gvStatusMaster_RowDataBound(object sender, GridViewRowEventArgs e)
{
if (e.Row.RowType == DataControlRowType.DataRow)
{
DropDownList ddlfilter = (e.Row.FindControl("ddlSM_Filter") as DropDownList);
ddlfilter.DataSource = stbal.LoadFilterBAL();
ddlfilter.DataTextField = "SM_Table_Filter";
ddlfilter.DataValueField = "SM_Table_Filter";
ddlfilter.Items.Insert(0, new ListItem("--Please Select--"));
string strfilter = (e.Row.FindControl("lblsm_filter") as Label).Text;
ddlfilter.Items.FindByValue(strfilter).Selected = true;
}
}