无法将ImageView添加到后台JavaFX -没有fxml

<!-- sample.fxml -->
<?xml version="1.0" encoding="UTF-8"?>

<?import javafx.scene.layout.VBox?>
<?import javafx.scene.image.ImageView?>

<VBox xmlns="http://javafx.com/javafx/15.0.1" xmlns:fx="http://javafx.com/fxml/1" fx:controller="com.example.SampleController">
    <ImageView fx:id="imageView" fitWidth="200" fitHeight="200" />
</VBox>

// SampleController.java
package com.example;

import javafx.fxml.FXML;
import javafx.scene.image.ImageView;

public class SampleController {
    @FXML
    private ImageView imageView;

    // 其他代码...
}

// MainApp.java
package com.example;

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

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

        // 其他代码...

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

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