Simple application with Combo Box gives Communication Error


I am facing an issue with a very basic application having just a combo box in it. When I click on the arrow button of the combo box, it throws communciation error.

I first tried to search in the forum and found a similar topic, but i could not open that topic because of wrong link.

Please find the attached screenshot for the same topic.

Source code:
public class TestUI extends UI

@WebServlet(value = “/Combo”, asyncSupported = true)
@VaadinServletConfiguration(productionMode = false, ui = TestUI.class)
public static class Servlet extends VaadinServlet

protected void init(VaadinRequest request)

Layout layout = new VerticalLayout();
ComboBox select = new ComboBox("Select an item");
select.addItem("Item 001");
select.addItem("Item 002");


I think I found it.

@WebServlet(urlPatterns = {“/Combo/", "/VAADIN/”}, asyncSupported = true)

The above configuration solves it.