private void hesapAçmaToolStripMenuItem_Click(object sender, EventArgs e)
 {
     if (HesapAcma == null)
     {
         HesapAcma             = new frm_HesapAcma(musteriBanka);
         HesapAcma.MdiParent   = this;
         HesapAcma.FormClosed += HesapAcma_FormClosed;
         HesapAcma.Show();
     }
 }
 private void HesapAcma_FormClosed(object sender, FormClosedEventArgs e)
 {
     HesapAcma = null;
 }