code: public void startGui(final Stage mainStage) {
          try {
              onStageCreated(mainStage);
              final Scene mainScene = getScene(mainComponent());
              onSceneCreated(mainScene);
              mainStage.setScene(mainScene);
              mainStage.setTitle(title());
  
              Option.ofOptional(getStylesheet())
                    .filter(style -> !FxmlStylesheets.DEFAULT_JAVAFX_STYLE.equals(style))
                    .peek(stylesheet -> this.setTheme(stylesheet, mainStage));
  
              mainStage.show();
          } catch (Throwable t) {
              LOGGER.error("There was a failure during start-up.", t);
          }
      }
docs: Called by {@link FxApplication} after Spring and JavaFX are started. This is the equivalent of {@link
  javafx.application.Application#start(Stage)} in traditional JavaFX apps.
  
  @param mainStage A reference to the main stage of the application, received from JavaFX via {@link javafx.application.Application#start(Stage)}.