[SOLUCIONADO] Ayuda usar JFrame!!!

import javafx.application.Application;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;

public class MiPrograma extends Application {
    @Override
    public void start(Stage stage) {
	Button btn = new Button("Click me!");
	btn.setOnAction(new EventHandler<ActionEvent>() {
	    @Override
	    public void handle(ActionEvent event) {
		System.out.println("Hola mundo!");
	    }
	});
	StackPane root = new StackPane();
	root.getChildren().add(btn);
	Scene scene = new Scene(root, 300, 250);
	stage.setTitle("Demo JavaFX para Underc0de");
	stage.show();
    }
    public static void main(String[] args) {
	launch(args); // llama internamente a start
    }
}

<?xml version="1.0" encoding="UTF-8"?>

<?import javafx.scene.effect.*?>
<?import javafx.scene.image.*?>
<?import javafx.geometry.*?>
<?import javafx.scene.text.*?>
<?import javafx.scene.control.*?>
<?import java.lang.*?>
<?import javafx.scene.layout.*?>

<AnchorPane prefHeight="443.0" prefWidth="908.0" stylesheets="@css/style.css" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1" fx:controller="me.localtest.customersapp.controllers.MainController">
   <children>
       <!-- aquí pueden ir botones, tablas, textfields, ect -->
       <Button fx:id="btnGreet" text="Click me!" layoutX="100" layoutY="35" onAction="greetAction"/>
   </children>
</AnchorPane>

public class MainController {
    @FXML Button btnGreet;
	
    @FXML
    private void greetAction() {
	System.out.println("Hola mundo!");
    }
}