JavaFX - getScene() returns null

Question

I just started using JavaFX Scene Builder to build a small application.

It is made up of a controller class 'Login.java' which belongs to 'login.fxml', in which the FXML file 'registrierung.fxml' is loaded via a method called 'registrationClicked(ActionEvent event)':

public class Login {

@FXML
private void registrationClicked(ActionEvent event){
    try{
        ((Node) (event.getSource())).getScene().getWindow().hide();
        FXMLLoader loader = new FXMLLoader(getClass().getResource("/view/fxml/registrierung.fxml"));
        Parent root = (Parent) loader.load();
        Stage stage = new Stage();
        Scene scene = new Scene(root);      
        stage.setTitle("Registration");
        stage.setScene(scene);
        stage.setResizable(false);
        stage.show();
    } catch(IOException e){
        e.printStackTrace();
    }
}

Now I want to get a reference to the stage of 'registrierung.fxml' in the controller class 'Registrierung.java' via the root node vboxRoot:

@FXML
private VBox vboxRoot;

Stage stage = (Stage) vboxRoot.getScene().getWindow();

However 'getScene()' always leads to a NullPointerException. The controller classes for both FXML files are adjusted in Scene Builder.

This is how I set up the rood node in 'registrierung.fxml':

<VBox fx:id="vboxRoot" maxHeight="-Infinity" maxWidth="-Infinity" prefHeight="267.0" prefWidth="355.0" stylesheets="@../css/styles.css" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1" fx:controller="businesslogik.Registrierung">

What am I doing wrong?