private void ShowNodeList_Checked(object sender, RoutedEventArgs e)
 {
     if (nodeList == null)
     {
         nodeList = new NodeListWindow(this, airfield);
     }
     nodeList.Closed += NodeList_Closed;
     nodeList.Show();
 }
 private void NodeList_Closed(object sender, EventArgs e)
 {
     nodeList = null;
 }