Exception when starting a new scene

[alessiapacca]

Everything is ok until I try to visualize the java fx scene:

        // create a flow object
        VFlow flow = FlowFactory.newFlow();

        // add two nodes to the flow
        VNode n1 = flow.newNode();
        VNode n2 = flow.newNode();

        // create input and output connectors of type "default-type"
        Connector inN1 = n1.addInput("default-type");
        Connector outN1 = n1.addOutput("default-type");
        Connector inN2 = n2.addInput("default-type");
        Connector outN2 = n2.addOutput("default-type");

        // create a connections
        flow.connect(outN1, inN2);
        // we assume a flow already exists
        // make the flow visible
        flow.setVisible(true);

        // create a zoomable canvas
        VCanvas canvas = new VCanvas();
        Pane root = (Pane) canvas.getContent();

        // creating a skin factory and attach it to the flow
        FXSkinFactory skinFactory = new FXSkinFactory(root);
        flow.setSkinFactories(skinFactory);

        Scene scene = new Scene(canvas);
        primaryStage.setTitle("Try");
        primaryStage.setScene(scene);
        primaryStage.show();// create a flow object
        VFlow flow = FlowFactory.newFlow();

        // add two nodes to the flow
        VNode n1 = flow.newNode();
        VNode n2 = flow.newNode();

        // create input and output connectors of type "default-type"
        Connector inN1 = n1.addInput("default-type");
        Connector outN1 = n1.addOutput("default-type");
        Connector inN2 = n2.addInput("default-type");
        Connector outN2 = n2.addOutput("default-type");

        // create a connections
        flow.connect(outN1, inN2);
        // we assume a flow already exists
        // make the flow visible
        flow.setVisible(true);

        // create a zoomable canvas
        VCanvas canvas = new VCanvas();
        Pane root = (Pane) canvas.getContent();

        // creating a skin factory and attach it to the flow
        FXSkinFactory skinFactory = new FXSkinFactory(root);
        flow.setSkinFactories(skinFactory);

        Scene scene = new Scene(canvas);
        primaryStage.setTitle("Try");
        primaryStage.setScene(scene);
        primaryStage.show();

I get the following error:

Exception in Application start method
java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at com.sun.javafx.application.LauncherImpl.launchApplicationWithArgs(LauncherImpl.java:389)
    at com.sun.javafx.application.LauncherImpl.launchApplication(LauncherImpl.java:328)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at sun.launcher.LauncherHelper$FXHelper.main(LauncherHelper.java:767)
Caused by: java.lang.RuntimeException: Exception in Application start method
    at com.sun.javafx.application.LauncherImpl.launchApplication1(LauncherImpl.java:917)
    at com.sun.javafx.application.LauncherImpl.lambda$launchApplication$154(LauncherImpl.java:182)
    at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.NullPointerException
    at jfxtras.scene.control.window.Window.getUserAgentStylesheet(Window.java:501)
    at com.sun.javafx.css.StyleManager.findMatchingStyles(StyleManager.java:1683)
    at javafx.scene.CssStyleHelper.createStyleHelper(CssStyleHelper.java:111)
    at javafx.scene.Node.reapplyCss(Node.java:8985)
    at javafx.scene.Node.reapplyCss(Node.java:9014)
    at javafx.scene.Node.reapplyCss(Node.java:9014)
    at javafx.scene.Node.impl_reapplyCSS(Node.java:8948)
    at javafx.scene.Node.invalidatedScenes(Node.java:856)
    at javafx.scene.Node.setScenes(Node.java:921)
    at javafx.scene.Scene$9.invalidated(Scene.java:1119)
    at javafx.beans.property.ObjectPropertyBase.markInvalid(ObjectPropertyBase.java:111)
    at javafx.beans.property.ObjectPropertyBase.set(ObjectPropertyBase.java:146)
    at javafx.scene.Scene.setRoot(Scene.java:1072)
    at javafx.scene.Scene.<init>(Scene.java:347)
    at javafx.scene.Scene.<init>(Scene.java:194)
    at sample.Main.start(Main.java:109)
    at com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$161(LauncherImpl.java:863)
    at com.sun.javafx.application.PlatformImpl.lambda$runAndWait$174(PlatformImpl.java:326)
    at com.sun.javafx.application.PlatformImpl.lambda$null$172(PlatformImpl.java:295)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.application.PlatformImpl.lambda$runLater$173(PlatformImpl.java:294)
    at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
    at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at com.sun.glass.ui.win.WinApplication.lambda$null$147(WinApplication.java:177)
    ... 1 more
Exception running application sample.Main

which is triggered by the new Scene(canvas) line

did this happen to anybody else?