void dat_FormClosed(object sender, FormClosedEventArgs e)
 {
     dat = null;
 }
        private void deleteATrainingInformationToolStripMenuItem_Click(object sender, EventArgs e)
        {
            if (dat== null)
            {
                dat = new DeleteATraining(n);
                dat.MdiParent = this;
                dat.FormClosed += dat_FormClosed;
                dat.Show();

            }
            else
            {
                dat.Activate();
            }

        }