将子元素添加到java fxml中fx:root元素的子元素中

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

<?import javafx.scene.layout.VBox?>
<?import javafx.scene.control.Button?>

<VBox xmlns="http://javafx.com/javafx/15.0.1" xmlns:fx="http://javafx.com/fxml/1" fx:controller="com.example.Controller">
    <Button text="子元素1" fx:id="button1" />
    <Button text="子元素2" fx:id="button2" />
</VBox>

package com.example;

import javafx.fxml.FXML;
import javafx.scene.control.Button;
import javafx.scene.layout.VBox;

public class Controller {
    @FXML
    private VBox root;

    public void initialize() {
        Button button3 = new Button("子元素3");
        root.getChildren().add(button3);
    }
}

package com.example;

import javafx.application.Application;
import javafx.fxml.FXMLLoader;
import javafx.scene.Scene;
import javafx.stage.Stage;

public class MainApp extends Application {
    @Override
    public void start(Stage primaryStage) throws Exception {
        FXMLLoader loader = new FXMLLoader(getClass().getResource("Root.fxml"));
        VBox root = loader.load();
        Controller controller = loader.getController();
        controller.initialize();

        Scene scene = new Scene(root, 400, 300);
        primaryStage.setScene(scene);
        primaryStage.show();
    }

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