private void btnThem_Click(object sender, EventArgs e)
        {
            MoDieuKien();
            SetNull();
            themmoi = true;
            int ma;

            if (bus.CountID() == 1)
            {
                ma = (int.Parse(bus.GetLastID()) + 1);
            }
            else
            {
                ma = 0;
            }
            txtMaNV.Text = ma.ToString();
        }