EventType "ADD_SENSOR" with parent "EVENT" already exists

Question

I am developing a JavaFX application which uses the Apache Felix OSGi framework. I have an issue with firing custom events. Whenever I try to fire a custom event, the exception java.lang.IllegalArgumentException: EventType "ADD_SENSOR" with parent "EVENT" already exists (stacktrace is shown below) is thrown.

Caused by: java.lang.IllegalArgumentException: EventType "ADD_SENSOR"with parent "EVENT" already exists
    at javafx.event.EventType.register(EventType.java:186)
    at javafx.event.EventType.<init>(EventType.java:128)
    at swt6.componentmanager.gui.event.AddSensorEvent.<clinit>(AddSensorEvent.java:8)

The code firing the event is shown below.

private void onAddSensor(ActionEvent event) {
    AddSensorEvent addSensorEvent = new AddSensorEvent(
            this.typeComboBox.getValue(),
            this.nameInput.getText(),
            this.descriptionInput.getText()
    );
    this.addButton.fireEvent(addSensorEvent);

    // close the window
    ((Stage) this.getScene().getWindow()).close();
}

The class AddSensorEvent, which also manages the EventType instance via a public static final field, is illustrated below, too.

public class AddSensorEvent extends Event {
    public static final EventType<AddSensorEvent> ADD_SENSOR_EVENT_TYPE =
            new EventType<>(EventType.ROOT, "ADD_SENSOR");

    private final SensorFactory sensorFactory;
    private final String sensorName;
    private final String sensorDescription;

    public AddSensorEvent(SensorFactory sensorFactory, String sensorName, String sensorDescription) {
        super(ADD_SENSOR_EVENT_TYPE);

        this.sensorFactory = sensorFactory;
        this.sensorName = sensorName;
        this.sensorDescription = sensorDescription;
    }

    public SensorFactory getSensorFactory() {
        return this.sensorFactory;
    }

    public String getSensorName() {
        return this.sensorName;
    }

    public String getSensorDescription() {
        return this.sensorDescription;
    }
}

The application is a port of an application using the Java module system, which was introduced in Java 9, and it works fine when using the module system.

I tried following this, this and this issues, but none of the provided solutions worked for me.

Thank you for your help!