alert box is not working in javascript while validating html form in JAVAFX

i am pretty much new to javafx. i have built an application on javafx which will load index.html page through WebView. when i click on register button in index.html new registration form page will be loaded.

javafx code:

public void start(final Stage primaryStage) throws Exception {
    primaryStage.setResizable(false);
    FrontEndFinal f=new FrontEndFinal();

    final WebView browser = new WebView();
    final WebEngine webEngine = browser.getEngine();
    webEngine.setJavaScriptEnabled(true);
    Worker<Void> worker = webEngine.getLoadWorker();
    worker.stateProperty().addListener(new ChangeListener<State>() {

        public void changed(ObservableValue<? extends State> observable, //
                State oldValue, State newValue) {

            // When load successed.
            if (newValue == Worker.State.SUCCEEDED) {

                JSObject jsobj = (JSObject) webEngine.executeScript("window");

                jsobj.setMember("myJavaMember", new FrontEndFinal());
            }
        }
    });
    VBox root = new VBox();

    root.setMinHeight(500);
    root.setMinWidth(1250);


    System.out.println(System.getProperty("user.dir"));
    String p=System.getProperty("user.dir");
    String [] t=p.split("\\\\");
    System.out.println(t.length);
    String final1=t[0]+"/";
    for(int i=1;i<t.length;i++)
    {
        final1=final1+t[i]+"/";
    }
    System.out.println(final1);


    browser.getEngine().load("file:///"+final1+"/Html"+"/SMS.html");
    root.getChildren().add(browser); 
}

When I run this code it loads index.html

in Index.html:

<button class="dropbtn"  onclick="location.href = 'Registration.html'"><b>< Register ></b></button>

after clicking button it opens Registration.html

In Registration.html

<form name="myForm"  method="post">
Name: <input type="text" name="fname">
<button type="button" name="Submit" onclick="abc()">Submit</button>
<script>
function abc(){
    window.alert("Invalid name");
}

Now my problem is when i submit Registration.html form with name field blank, it does not shows alert box i googled many sites like:[https://community.oracle.com/thread/2540279] but didn't work Any help or suggestions?

Thanks In Advance