我是javafx新手，我正在尝试建立一个通过menuitem动作加载新场景的项目。
这是fxml菜单（“已使用场景生成器创建所有fxml”）：

<HBox alignment="CENTER_RIGHT" prefHeight="50.0" prefWidth="200.0">
    <children>
        <Button fx:id="btnLogin" mnemonicParsing="false" onAction="#btnLoginClicked" text="Login" /> 
        <MenuButton fx:id="btnMenu" alignment="CENTER_RIGHT" mnemonicParsing="false" text="MenuButton">
            <items>
                <MenuItem fx:id="btnLoadArticle" mnemonicParsing="false" onAction="#btnLoadArticleClicked" text="Load article from file" />
                <MenuItem fx:id="btnNewArticle" mnemonicParsing="false" onAction="#btnNewArticleClicked" text="New Article" />
                <MenuItem fx:id="btnEditArticle" mnemonicParsing="false" onAction="#btnEditArticleClicked" text="EditArticle" />
                <MenuItem fx:id="btnDeleteArticle" mnemonicParsing="false" onAction="#btnDeleteArticleClicked" text="Delete article" />
                <MenuItem fx:id="btnExit" mnemonicParsing="false" onAction="#btnExitClicked" text="Exit" />
            </items>
            <HBox.margin>
                <Insets bottom="20.0" left="20.0" right="20.0" top="20.0" />
            </HBox.margin>
        </MenuButton>
    </children>
    <VBox.margin>
        <Insets bottom="10.0" left="20.0" right="20.0" />
    </VBox.margin>
</HBox>

这是单击“btnediticle”时的控制器代码（其中显示菜单）：

@FXML
private void btnEditArticleClicked(ActionEvent event) {
    if (selectedArticle != null) {
        try {
            ArticleEditController editController = new ArticleEditController(this);
            editController.setArticle(selectedArticle);

            MenuItem eventOrigin = (MenuItem) event.getSource();
            ContextMenu contextMenu = eventOrigin.getParentPopup();
            contextMenu.getOwnerWindow().getScene().setRoot(editController.getContent());
        } catch (Exception e) {
            e.printStackTrace();
        }
    } else {
        Alert alert = new Alert(AlertType.INFORMATION);
        alert.setTitle("Want to edit an article?");
        alert.setHeaderText(null);
        alert.setContentText("An article from the list must be selected in order to edit it.");
        alert.showAndWait();
    }
}

这是我在场景改变时执行的代码：

private Pane root;

public ArticleEditController(NewsReaderController newsReaderController) {
    this.newsReaderController = newsReaderController;

    try {
        FXMLLoader loader = new FXMLLoader(getClass().getResource(AppScenes.EDITOR.getFxmlFile()));
        loader.setController(this);
        root = loader.load();
    } catch (IOException e) {
        e.printStackTrace();
    }
}

public Pane getContent() {
    return root;
}

当创建场景时，我得到一个nullpointerexception，但是之后内容会毫无问题地显示出来。这是我得到的唯一信息：

Exception in thread "JavaFX Application Thread" java.lang.NullPointerException
        at java.base/java.util.Objects.requireNonNull(Objects.java:222)
        at javafx.scene.control.skin.MenuButtonSkinBase.lambda$new$7(MenuButtonSkinBase.java:206)
        at com.sun.javafx.application.PlatformImpl.lambda$runLater$10(PlatformImpl.java:428)
        at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
        at com.sun.javafx.application.PlatformImpl.lambda$runLater$11(PlatformImpl.java:427)
        at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:96)
        at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
        at com.sun.glass.ui.win.WinApplication.lambda$runLoop$3(WinApplication.java:174)
        at java.base/java.lang.Thread.run(Thread.java:832)

希望你们任何人都能帮我。我已经检查过这个问题了，但是给出的答案和我需要的不一样。
提前谢谢！
编辑我使用main.java来显示如下场景：

@Override
public void start(Stage primaryStage) {     
    newsReaderController = new NewsReaderController();
    newsReaderController.setMainController(this);
    Pane root = newsReaderController.getContent();
    setConnection();

    Scene scene = new Scene(root);
    primaryStage.setTitle("Newspaper");
    primaryStage.initStyle(StageStyle.DECORATED);
    primaryStage.setScene(scene);
    primaryStage.show();
}