Icons not displayed in Gluon Desktop

Question

I'm new to Gluon mobile and desktop. I started a new project and built it successfully for android and desktop. It ran well without a hitch on the android device. However when I ran it on desktop, the Icons did not appear instead I got a rectangle where they should be. Below is the code for the fxml document.

<?import com.gluonhq.charm.glisten.control.Icon?>
<?import com.gluonhq.charm.glisten.mvc.View?>
<?import javafx.scene.control.Button?>
<?import javafx.scene.control.Label?>
<?import javafx.scene.effect.DropShadow?>
<?import javafx.scene.layout.BorderPane?>
<?import javafx.scene.layout.VBox?>

<View fx:id="start" maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="600.0" prefWidth="350.0" xmlns="http://javafx.com/javafx/8.0.141" xmlns:fx="http://javafx.com/fxml/1" fx:controller="com.id3editor.views.StartPresenter">
   <center>
      <VBox alignment="CENTER" prefHeight="200.0" prefWidth="100.0" spacing="15.0" BorderPane.alignment="CENTER">
         <children>
            <Label fx:id="label" text="Hello JavaFX World!" />
            <Button mnemonicParsing="false" onAction="#buttonClick" stylesheets="@start.css" text="Change the World!">
               <graphic>
                  <Icon content="LANGUAGE">
                     <effect>
                        <DropShadow />
                     </effect></Icon>
               </graphic>
            </Button>
         </children>
      </VBox>
   </center>
</View>

And this is what it looks like on my desktop:

Please I really hope this questions can be answered. I checked and made sure there was no question like it. My apologies if this ends up being a duplicate. Thanks. Here is the build.gradle file

buildscript {
    repositories {
        jcenter()
    }
    dependencies {
        classpath 'org.javafxports:jfxmobile-plugin:1.3.10'
    }
}

apply plugin: 'org.javafxports.jfxmobile'

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

mainClassName = 'com.id3editor.Id3editor'

dependencies {
    compile 'com.gluonhq:charm:4.4.1'

    compile 'com.airhacks:afterburner.mfx:1.6.3'
}

jfxmobile {
    downConfig {
        version = '3.6.0'
        // Do not edit the line below. Use Gluon Mobile Settings in your project context menu instead
        plugins 'display', 'lifecycle', 'statusbar', 'storage'
    }
    android {
        manifest = 'src/android/AndroidManifest.xml'
    }
    ios {
        infoPList = file('src/ios/Default-Info.plist')
        forceLinkClasses = [
                'com.id3editor.**.*',
                'com.gluonhq.**.*',
                'javax.annotations.**.*',
                'javax.inject.**.*',
                'javax.json.**.*',
                'org.glassfish.json.**.*'
        ]
    }
}

And I get these warnings when running

Gtk-Message: Failed to load module "unity-gtk-module"
Jan 17, 2018 12:40:13 PM com.gluonhq.charm.down.Platform <clinit>
SEVERE: javafx.platform is not defined. Desktop will be assumed by default.
Jan 17, 2018 12:40:14 PM com.sun.javafx.css.StyleManager loadStylesheetUnPrivileged
INFO: Could not load @font-face font [gls:file:/root/.gradle/caches/modules-2/files-2.1/com.gluonhq/charm-glisten/4.4.1/7bdb2fab385f250b2821c206ea669ff65a1c9ad6/charm-glisten-4.4.1.jar!/com/gluonhq/charm/glisten/control/MaterialIcons-Regular.ttf]
Jan 17, 2018 12:40:14 PM com.sun.javafx.css.StyleManager loadStylesheetUnPrivileged
INFO: Could not load @font-face font [gls:file:/root/.gradle/caches/modules-2/files-2.1/com.gluonhq/charm-glisten/4.4.1/7bdb2fab385f250b2821c206ea669ff65a1c9ad6/charm-glisten-4.4.1.jar!/com/gluonhq/charm/glisten/control/Roboto-Regular.ttf]
Jan 17, 2018 12:40:14 PM com.sun.javafx.css.StyleManager loadStylesheetUnPrivileged
INFO: Could not load @font-face font [gls:file:/root/.gradle/caches/modules-2/files-2.1/com.gluonhq/charm-glisten/4.4.1/7bdb2fab385f250b2821c206ea669ff65a1c9ad6/charm-glisten-4.4.1.jar!/com/gluonhq/charm/glisten/control/Roboto-Bold.ttf]
Jan 17, 2018 12:40:15 PM com.sun.javafx.css.StyleManager loadStylesheetUnPrivileged
INFO: Could not load @font-face font [gls:file:/root/.gradle/caches/modules-2/files-2.1/com.gluonhq/charm-glisten/4.4.1/7bdb2fab385f250b2821c206ea669ff65a1c9ad6/charm-glisten-4.4.1.jar!/com/gluonhq/charm/glisten/control/Roboto-Medium.ttf]

I am building and running with a 32-bit Linux.