...

public class MessageViewerPart {

  private MessagePane             msgPane;

  @Inject
  private MessageViewerModel      model;
  @Inject
  private MessageViewerController controller;

  @PostConstruct
  public void init(final MApplication app, BorderPane parent) {
    parent.setPadding(new Insets(5));
    parent.setCenter(createPane());

    bindControls();
  }

  private Node createPane() {
    BorderPane pane = new BorderPane();

    msgPane = new MessagePane();
    pane.setCenter(msgPane);

    pane.setBottom(createControlsPane());

    return pane;
  }

  private Node createControlsPane() {
    HBox pane = new HBox();
    pane.setSpacing(5);
    pane.setAlignment(Pos.CENTER);

    Button navFirstButton = new Button("First");
    pane.getChildren().add(navFirstButton);

    navFirstButton.setOnAction(new EventHandler<ActionEvent>() {
      @Override
      public void handle(ActionEvent event) {
        MessageDialog.openInformationDialog(<<need Window reference>>, "Title", "Message");
      }
    });

    return pane;
  }

...