How to properly use Icon in Gluon Scene Builder 11?

Question

I'm trying to create a javafx desktop app. For this I use Java 13, JavaFX 15 and SceneBuilder 11. I have already followed the guide on how to set-up a javafx project with gradle in Intellij from here: https://openjfx.io/openjfx-docs/ ( JavaFX and Intellij -> Non-modular with Gradle ). After the set-up is done, I opened the .fxml file in SceneBuilder, added an Icon from the Gluon sub-menu and added some styling to it, like in this picture: scene builder picture
However, when running the application there is a rectangle displayed instead of the selected Icon, like in this picture: application picture
This is my gradle file, in case the problem is related to the dependencies:

plugins {
    id 'application'
    id 'org.openjfx.javafxplugin' version '0.0.8'
}

repositories {
    maven {
        url 'https://nexus.gluonhq.com/nexus/content/repositories/releases/'
    }
    mavenCentral()
}

dependencies {

    implementation 'com.gluonhq:charm-glisten:6.0.5'
}

javafx {
    version = "15.0.1"
    modules = ['javafx.controls', 'javafx.fxml']
}

mainClassName = 'org.stoicescu.MainApp'

Also this is my .fxml file:

<?xml version="1.0" encoding="UTF-8"?>

<?import com.gluonhq.charm.glisten.control.Icon?>
<?import javafx.scene.layout.BorderPane?>
<?import javafx.scene.layout.HBox?>

<BorderPane maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="400.0"
            prefWidth="600.0" xmlns="http://javafx.com/javafx/11.0.1" xmlns:fx="http://javafx.com/fxml/1"
            fx:controller="org.stoicescu.FXMLController">
    <top>
        <HBox prefHeight="40.0" prefWidth="600.0" BorderPane.alignment="CENTER">
            <children>
                <Icon content="ASSISTANT_PHOTO" prefHeight="40.0" prefWidth="40.0"
                      style="-fx-background-color: green;"/>
            </children>
        </HBox>
    </top>
</BorderPane>

What could be the problem?