private void btnQLHĐ_Click(object sender, EventArgs e)
        {
            QuanlyHoadon quanlyHoadon = new QuanlyHoadon();

            quanlyHoadon.Show();
        }
        private void invoicesToolStripMenuItem_Click(object sender, EventArgs e)
        {
            QuanlyHoadon quanlyHoadon = new QuanlyHoadon();

            addForm(quanlyHoadon);
        }