Client don't receive Data sent by server in "onopen"

Hi everybody
I'm using websockets for developping a chat application with jee, I'm using java in backend and javascript in frontend. and when opening the websocket and want to get the id session from the server and use it in my client page.

Here is the code i was tried:

Server side:

@ServerEndpoint("/echo")
public class LiveStream {
 
     @OnOpen
    public void onOpen(Session session) {
        try {
            session.getBasicRemote().sendText(session.getId());
        } catch (IOException ex) {
        }
         
        session.setMaxBinaryMessageBufferSize(1024 * 512);
         
    }
 
    @OnMessage
    public void processVideo(byte[] imageData, Session session) {
        try {
            ByteBuffer buf = ByteBuffer.wrap(imageData);
            Set set;
            set = session.getOpenSessions();
            System.out.println(set.size() + " this is the number of opened sessions");
            Iterator it = set.iterator();
            while (it.hasNext()) {
                Session s = (Session) it.next();
                if(s!=session){
                s.getBasicRemote().sendBinary(buf);
                }
            }
 
        } catch (IOException ex) {
        } catch (Throwable ioe) {
            System.out.println("Error sending message " + ioe.getMessage());
        }
    }
 
    @OnClose
    public void whenClosing(Session session) {
        System.out.println("Goodbye !");
        sessions.remove(session);
    }
 
}

Client side:

<script>
           
            window.onload = function () {
                ws = new WebSocket("ws://localhost:8080/application/echo");
                ws.onopen = function (event) {
                    if (event.data === undefined)  return;
                    id = event.data;
                    document.getElementById('texxt').value += id;
                    };
 
               ws.onmessage = function (msg) {
                    url = window.URL.createObjectURL(msg.data);
                  };
                            
            }

The problem is that the id session is not received from the server and in client side, event.data===undefined. and I have to access to id_session of each user to use it later.

I need your help please ans thank you