控制器节点在setOnCloseRequest中是否始终为null？

Question

我有以下代码：

public class Main extends Application {

    @FXML
    private Button button;

    @Override
    public void start(Stage primaryStage) {
        try {
            ...

            primaryStage.setOnCloseRequest(new EventHandler<WindowEvent>() {

                @Override
                public void handle(WindowEvent event) {
                    System.out.println(button)
                }

            });

            ...
    }

    @FXML
    private void initialize() {
        System.out.println(button);
    }

}

我运行它，然后点击关闭按钮，这是输出：

Button[id=button, styleClass=button]'button'
null

为什么在setOnCloseRequest中调用按钮时为null？