private void btThemKM_Click(object sender, EventArgs e)
 {
     FrmKhuyenMai fr = new FrmKhuyenMai();
     fr.ShowDialog();
 }
 private void btKhuyenMai_Click(object sender, EventArgs e)
 {
     FrmKhuyenMai fr = new FrmKhuyenMai();
     fr.Show();
 }