How to add a package controller to a project created with FXML JavaFX Maven Archetype (Gluon) in Netbeans 12?

Question

When I create a project with FXML JavaFX Maven Archetype (Gluon) in Netbeans 12, I get the following hierarchy of files in the project:

└── helloworld
    ├── App.java
    ├── PrimaryController.java
    └── SecondaryController.java

This is all right, but I would like to get the following structure, instead:

└── helloworld
    └── controller
        ├── App.java
        ├── PrimaryController.java
        └── SecondaryController.java

└── resources
    └── com
        └── whatever
            └── fx
                └── helloworld
                    └── fxml
                        ├── primary.fxml
                        └── secondary.fxml

I follow these steps:

1. Add a package controller to helloworld package.
   Add a package fxml in helloworld package of resources

2. Put App.java, PrimaryController.java, and SecondaryController.java in the package controller.
   Put primary.fxml and secondary.fxml in package fxml

Code Java

public class App extends Application {

    private static Scene scene;

    @Override
    public void start(Stage stage) throws IOException {
        scene = new Scene(loadFXML("fxml/primary"), 640, 480);
        stage.setScene(scene);
        stage.show();
    }

    public static void setRoot(String fxml) throws IOException {
        scene.setRoot(loadFXML(fxml));
    }

    private static Parent loadFXML(String fxml) throws IOException {
        FXMLLoader fxmlLoader = new FXMLLoader(App.class.getResource(fxml + ".fxml"));
        return fxmlLoader.load();
    }

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

public class PrimaryController {

    @FXML
    private void switchToSecondary() throws IOException 
    {
        App.setRoot("fxml/secondary");
    }
}

public class SecondaryController {

    @FXML
    private void switchToPrimary() throws IOException {
        App.setRoot("fxml/primary");
    }

3. In Module-info.java, replace

opens com.whatever.fx.helloworld to javafx.fxml;
exports com.whatever.fx.helloworld;

with

opens com.whatever.fx.helloworld.controller to javafx.fxml;
exports com.whatever.fx.helloworld.controller;

4. In pom.xml, replace

<configuration>
    <mainClass>com.whatever.fx.helloworld.App</mainClass>
</configuration>

with

<configuration>
    <mainClass>com.whatever.fx.helloworld.controller.App</mainClass>
</configuration>

5. In primary.fxml, replace

fx:controller="com.whatever.fx.helloworld.PrimaryController">

with

fx:controller="com.whatever.fx.helloworld.controller.PrimaryController">

6. In secondary.fxml, replace

x:controller="com.whatever.fx.helloworld.SecondaryController">

with

fx:controller="com.whatever.fx.helloworld.controller.SecondaryController">

However, after following all of the above steps, I get the following error:

Exception in Application start method
java.lang.reflect.InvocationTargetException
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:564)
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.launchApplicationWithArgs(LauncherImpl.java:464)
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.launchApplication(LauncherImpl.java:363)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:564)
    at java.base/sun.launcher.LauncherHelper$FXHelper.main(LauncherHelper.java:1051)
Caused by: java.lang.RuntimeException: Exception in Application start method
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.launchApplication1(LauncherImpl.java:900)
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.lambda$launchApplication$2(LauncherImpl.java:195)
    at java.base/java.lang.Thread.run(Thread.java:832)
Caused by: java.lang.IllegalStateException: Location is not set.
    at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2459)
    at javafx.fxml/javafx.fxml.FXMLLoader.load(FXMLLoader.java:2435)
    at com.whatever.fx.helloworld/com.whatever.fx.helloworld.controller.App.loadFXML(App.java:31)
    at com.whatever.fx.helloworld/com.whatever.fx.helloworld.controller.App.start(App.java:20)
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$9(LauncherImpl.java:846)
    at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runAndWait$12(PlatformImpl.java:455)
    at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runLater$10(PlatformImpl.java:428)
    at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
    at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runLater$11(PlatformImpl.java:427)
    at javafx.graphics/com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:96)
    at javafx.graphics/com.sun.glass.ui.gtk.GtkApplication._runLoop(Native Method)
    at javafx.graphics/com.sun.glass.ui.gtk.GtkApplication.lambda$runLoop$11(GtkApplication.java:277)
    ... 1 more
Exception running application com.whatever.fx.helloworld.controller.App
Command execution failed.