This one starts from launcher  

    @Override
    public void start(Stage stage) throws IOException {
        { --- some irrelevant task specifics --- }
 
        FXMLLoader fxmlLoader = new FXMLLoader(GS_Application.class.getResource("GS_view.fxml"));
        /*
          the rootLayout of the mainStage is formatted as a <code>BorderPane</code>
         */
        BorderPane rootLayout = fxmlLoader.load();
        primaryStage = stage;
        scene0 = new Scene(rootLayout, 900, 800);
        Image _img = new Image(Objects.requireNonNull(this.getClass().getResourceAsStream("GS_MV.png")));
        stage.setTitle("G_String");
        stage.getIcons().add(_img);
        myNest.setScene(scene0);

        {---- some further task specifics ---}
         
        primaryStage.show();
    }

    public static void main(String[] args) {
        launch();
    }
------------------------------------------------------------------------------------------
This one doesn't!

    @Override
    public void start(Stage stage) throws IOException {
        FXMLLoader fxmlLoader = new FXMLLoader(ChainProMain.class.getResource("layouts/RootLayout.fxml"));
        BorderPane rootLayout = fxmlLoader.load();
        Scene scene = new Scene(rootLayout, 900, 700);
        stage.setTitle("ChainPro_m");
        stage.setScene(scene);
        primaryStage = stage;
        controller = fxmlLoader.getController();
        controller.setMainApp(this);

        { ---- some task specifics ---}

         primaryStage.show();
    }

    public static void main(String[] args) {
        launch();
    }