JavaFX in version 4

I am able to add Text, Rectangle, Circle to a JavaFX window in version 4.0.1, but fail when it comes to adding a Button control. Error msg is shown below. Does anyone know how to fix this?

import processing.javafx.*;  // Add this for Processing 4:

import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;

void setup() {  
  size(1, 1, FX2D);
  surface.setVisible(false); // Don't show default Processing window
  Stage stage = new Stage();
  stage.setTitle("Button demo");
  Button btn = new Button("button");
  StackPane pane = new StackPane();
  Scene scene = new Scene(pane, 400, 400);

void draw() {