private void agregarClienteToolStripMenuItem_Click(object sender, EventArgs e)
 {
     if (client == null)
     {
         client             = new frmAgregarClientes();
         client.MdiParent   = this;
         client.FormClosed += new FormClosedEventHandler(client_FormClosed);
         client.Show();
     }
 }
 void client_FormClosed(object sender, EventArgs e)
 {
     client = null;
 }