Experience with Server Push?

public class PushContent {
  
  private final Desktop desktop;

  public PushContent(Desktop aDesktop) {
    desktop = aDesktop;
  }
  
  public void notifyUsers() throws DesktopUnavailableException, InterruptedException {
    // note: push has been activated for the desktop at session startup (assuming there is only one desktop per session) 
    // nevertheless I get an illegal state exception and activate it again
    desktop.enableServerPush(true);
    Executions.activate(desktop);
    for ( int i = 0; i < 5; i++ ) {      
      try {
        Clients.showBusy("Executing " + (i + 1) + " of 5");
      } finally {
        Executions.deactivate(desktop);
      }
      Thread.sleep(2000);
    }
    desktop.enableServerPush(false);
  }
}

    final Desktop desktop = Executions.getCurrent().getDesktop();
    Runnable runnable = new Runnable() {
      @Override public void run() {
        try {
          new PushContent(desktop).notifyUsers();
        } catch ( DesktopUnavailableException e ) {
          e.printStackTrace();
        } catch ( InterruptedException e ) {
          e.printStackTrace();
        }
      }
    };
    Registry.threadPool().execute(runnable); // the thread pool is a standard executor service

public class SessionInitListener implements SessionInit {
  @Override
  public void init(Session session, Object request) throws Exception {
    // Enable AJAX push. This is required to notify users of external events.
    Executions.getCurrent().getDesktop().enableServerPush(true);

Exception in thread "pool-1-thread-2" java.lang.IllegalStateException: Server Push cannot be started without execution
	at org.zkoss.zk.ui.impl.DesktopImpl.enableServerPush0(DesktopImpl.java:1348)
	at org.zkoss.zk.ui.impl.DesktopImpl.enableServerPush(DesktopImpl.java:1336)
	at com.tn_ag.tnp.tailor.gui.push.PushContent.notifyUsers(PushContent.java:19)