private void vendasBtn1_Click(object sender, EventArgs e)
 {
     if (telaVenda == null)
     {
         telaVenda           = new TelaVenda();
         telaVenda.Disposed += new EventHandler(telaVenda_Disposed);
         telaVenda.Show();
     }
 }
 void telaVenda_Disposed(object sender, EventArgs e)
 {
     telaVenda = null;
 }