private void consultarClienteToolStripMenuItem_Click(object sender, EventArgs e)
 {    // ALTERAR CLIENTE
     if (telaAlterarCliente == null)
     {
         telaAlterarCliente = new Alterar_Cliente();
         telaAlterarCliente.Show();
         telaAlterarCliente.MdiParent = this;
         telaAlterarCliente.Disposed += new EventHandler(telaAlterarCliente_Disposed);
     }
 }
 void telaAlterarCliente_Disposed(object sender, EventArgs e)
 {
     telaAlterarCliente = null;
 }