private void nuevaVentaToolStripMenuItem_Click(object sender, EventArgs e)
 {
     if (ventas == null)
     {
         ventas             = new frmVerVentas();
         ventas.MdiParent   = this.MdiParent;
         ventas.FormClosed += new FormClosedEventHandler(VentasFormClosed);
         ventas.Show();
     }
 }
 private void VentasFormClosed(object sender, EventArgs e)
 {
     ventas = null;
 }