get the values from a Listt generic list

My Winforms C # project has a form with a GridControl(gvDados) component and a Process button with a Click event . The validation happens in the Click event of the process button, through a method responsible for returning true if there is a selected item and false if not later, that each item is stored in a generic list. p>

List<string> itensSelecionados;
.
.
.
public bool RetornaSelecionado(bool Selecionado)
{
  List<string> itensSelecionados = new List<string>();

  foreach (int i in gvDados.GetSelectedRows())
  {
    DataRow row = gvDados.GetDataRow(i);       
    itensSelecionados.Add(row[0].ToString());
    //MessageBox.Show(row[0].ToString());
  }
  if(linhas > 0)
  {
    MessageBox.Show("Selecionou " + itensSelecionados.Count()+ " Itens.");
    return Selecionado = true;
  }
  else
  {
    MessageBox.Show("Não selecionou");
    return Selecionado = false;
  }
}

My question is this: how can I also get all items saved in the itensSelecionados list in the click () event of the render button? I thought of doing it this way:

  foreach (var i in itensSelecionados)
  {
    MessageBox.Show(i);
  }

And I'm getting the following error: System.NullReferenceException