private void btnthithu_ItemClick(object sender, ItemClickEventArgs e)
        {
            Form frm = KiemTraTonTai(typeof(frmThi));

            if (frm != null)
            {
                frm.Activate();
            }
            else
            {
                frmThi f = new frmThi();
                f.MdiParent = this;
                f.Show();
            }
        }
        private void btnThiThu_ItemClick(object sender, DevExpress.XtraBars.ItemClickEventArgs e)
        {
            form = this.CheckExists(typeof(frmThi));
            if (form == null)
            {
                IsMdiContainer   = true;
                frmThi           = new frmThi();
                frmThi.MdiParent = this;

                frmThi.Show();
                checkOpenThiThu = true;
            }
            else
            {
                form.Activate();
            }
        }