JavaFX Enable and disable elements from outside

Question

I would like to access my Main Stage from outside. My mainstage is loaded with a fxml. After that I can press a login button on the mainstage and then a login popup opens. I want to set the button to Visi... as long as the login window is open. and then set it to Visi... afterwards. Furthermore I want to show several elements in the mainstage after my login was successful.

   @Override
   public void start(Stage primaryStage) throws Exception
   {
       Parent root = FXMLLoader.load(getClass().getResource("../resource/views/lobbyMain.fxml"));
       primaryStage.setTitle("Hello World");
       primaryStage.setScene(new Scene(root, 1000, 700));
       primaryStage.show();
       primaryStage.setOnCloseRequest(windowEvent -> {
           Platform.exit();
           if (loginStage != null){
               loginStage.close();
           }
           if (registrationStage != null){
               registrationStage.close();
           }
       });
   }

I hope you can help me in this regard. Thank you very much.