Why I'm unable to invoke Java methods from html content via the JavaScript call in JavaFX WebView?

I'm working on a task which needs to invoke a java method from a html content. This is a swing application and I used JavaFX WebView to load the HTML content into the application. But when I tried to invoke the Java methods it didn't work and sometime its giving fatal error and crashes the application.

Java class

class Solution extends JFrame { 
    
private JFXPanel jfxPanel;
static JFrame f; 

public static void main(String[] args) {
    new Solution().createUI();
}

    private void createUI() {
f = new JFrame("panel"); 

JPanel p = new JPanel(); 

jfxPanel = new JFXPanel();
createScene();
p.add(jfxPanel);

f.add(p);
f.setSize(300, 300); 
f.show(); 
    } 
    
    private void createScene() {
        
PlatformImpl.setImplicitExit(false);
PlatformImpl.runAndWait(new Runnable() {
@Override
public void run() {
BorderPane borderPane = new BorderPane();
WebView webComponent = new WebView();
WebEngine webEngine = webComponent.getEngine();

webEngine.load(TestOnClick.class.getResource("/mypage.html").toString());

borderPane.setCenter(webComponent);
Scene scene = new Scene(borderPane,300,300);
jfxPanel.setScene(scene);

JSObject window = (JSObject) webEngine.executeScript("window");
window.setMember("app", new Solution());
}
});
}
    
    public void onClick() {
        System.out.println("Invoked from JS");
    }
}

HTML

<button onclick="app.onClick()">Click ME</button>

Please let me know what needs to be changed here

From the documentation, both the class and method used for the callback must be public:

Calling back to Java from JavaScript

The JSObject.setMember method is useful to enable upcalls from JavaScript into Java code, as illustrated by the following example. The Java code establishes a new JavaScript object named app. This object has one public member, the method exit.

public class JavaApplication {
    public void exit() {
        Platform.exit();
    }
}
...
JavaApplication javaApp = new JavaApplication();
JSObject window = (JSObject) webEngine.executeScript("window");
window.setMember("app", javaApp);

...

The Java class and method must both be declared public.

(My emphasis.)

Your Solution class is not public, so this won't work.

In addition, when a new document is loaded, the window will lose its attributes. Since loading happens asynchronously, you need to ensure the member is set on the window after the document loads. You can do this via a listener on the documentProperty():

    webEngine.documentProperty().addListener((obs, oldDoc, newDoc) -> {
        JSObject window = (JSObject) webEngine.executeScript("window");
        window.setMember("app", this);          
    });

    webEngine.load(Solution.class.getResource("/mypage.html").toString());

There are a number of other problems with your code:

1. JFrames must be constructed on the AWT event dispatch thread (the same rule also applies to modifying components displayed in a JFrame). You can do this by wrapping the call to createUI() in SwingUtilities.invokeLater(...).
2. It is unclear why you made Solution a subclass of JFrame, as well as creating a new JFrame in createUI(). Since you never use the fact that Solution subclasses JFrame, you should remove that.
3. PlatformImpl is not part of the public API: consequently it would be perfectly OK for the JavaFX team to remove that class in a later release. You should use methods in the Platform class.
4. You almost certainly want the Javascript callback to interact with the current Solution instance, not some arbitrary instance you create. (If you're in an inner class, use Solution.this to access the current instance of the surrounding object.)

A working version of your code is

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;

import javafx.application.Platform;
import javafx.embed.swing.JFXPanel;
import javafx.scene.Scene;
import javafx.scene.layout.BorderPane;
import javafx.scene.web.WebEngine;
import javafx.scene.web.WebView;
import netscape.javascript.JSObject;

public class Solution  {

    private JFXPanel jfxPanel;

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Solution()::createUI);
    }



    private void createUI() {
        JFrame f = new JFrame("panel");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        JPanel p = new JPanel();

        jfxPanel = new JFXPanel();
        createScene();
        p.add(jfxPanel);

        f.add(p);
        f.setSize(300, 300);
        f.setVisible(true);
    }

    private void createScene() {

        Platform.setImplicitExit(false);
        Platform.runLater(() -> {
            BorderPane borderPane = new BorderPane();
            WebView webComponent = new WebView();
            WebEngine webEngine = webComponent.getEngine();

            webEngine.documentProperty().addListener((obs, oldDoc, newDoc) -> {
                JSObject window = (JSObject) webEngine.executeScript("window");
                window.setMember("app", this);          
            });

            webEngine.load(Solution.class.getResource("/mypage.html").toString());

            borderPane.setCenter(webComponent);
            Scene scene = new Scene(borderPane, 300, 300);
            jfxPanel.setScene(scene);

        });
    }

    public void onClick() {
        System.out.println("Invoked from JS");
    }

}