private void btnDangNhap_ItemClick(object sender, DevExpress.XtraBars.ItemClickEventArgs e)
        {
            CloseForm("Form9");
            FormDangNhap f = new FormDangNhap();

            f.MdiParent = this;
            f.Show();
        }
        private void đăngNhậpToolStripMenuItem_Click(object sender, EventArgs e)
        {
            FormDangNhap formDangNhap = new FormDangNhap();

            formDangNhap.Show(this);
        }