private void btHDX_Click(object sender, EventArgs e)
 {
     FrmHoaDonNhap fr = new FrmHoaDonNhap();
     fr.Show();
 }
 private void btHDN_Click(object sender, EventArgs e)
 {
     FrmHoaDonNhap fr = new FrmHoaDonNhap();
     fr.Sender(nhanvien.MaNV.ToString());
     fr.Show();
 }