private void DanhSachXa_Click_1(object sender, EventArgs e)
 {
     if (Xafrm == null || Xafrm.IsDisposed)
     {
         Xafrm           = new frmXa();
         Xafrm.MdiParent = this;
         Xafrm.Show();
     }
     else
     {
         Xafrm.Activate();
     }
 }