ComboBox does not display the dropdown list with items

I have a simple Liferay Portlet Vaadin application class:

public class TestApp extends Application implements
		PortletApplicationContext2.PortletListener {

	private Window fDisplayWindow;

	public void init() {
		fDisplayWindow = new Window("Test App");
		setMainWindow(fDisplayWindow);
		// Check that we are running as a portlet.
		if (getContext() instanceof PortletApplicationContext2) {
			PortletApplicationContext2 ctx = (PortletApplicationContext2) getContext();

			// Add a custom listener to handle action and
			// render requests.
			ctx.addPortletListener(this, this);
		} else {
			fDisplayWindow.showNotification("Not running in portal",
					Notification.TYPE_ERROR_MESSAGE);
		}
	}

	/**
	 * @see MVCPortlet#MVCPortlet()
	 */
	public TestApp() {
		super();
	}

	@Override
	public void handleResourceRequest(ResourceRequest aRequest,
			ResourceResponse aResponse, Window aWindow) {
		if (PortletMode.EDIT.equals(aRequest.getPortletMode())) fDisplayWindow
				.setContent(createEditContent());
		else if (PortletMode.VIEW.equals(aRequest.getPortletMode()))
			fDisplayWindow.setContent(createViewContent());
	}

	private ComponentContainer createEditContent() {
		Layout locResult = new VerticalLayout();
		locResult.addComponent(new ComboBox("Test to display",
				new IndexedContainer(Arrays.asList(new String[] { "one", "two",
						"three" }))));
		return locResult;
	}

	private ComponentContainer createViewContent() {
		Layout locResult = null;
		return locResult;
	}

	@Override
	public void handleRenderRequest(RenderRequest aRequest,
			RenderResponse aResponse, Window aWindow) {
	}

	@Override
	public void handleActionRequest(ActionRequest aRequest,
			ActionResponse aResponse, Window aWindow) {
	}

	@Override
	public void handleEventRequest(EventRequest aRequest,
			EventResponse aResponse, Window aWindow) {
	}

}

In this example everything is working fine after the portlet deployment, the combobox is displayed correctly in the EDIT mode of the portlet, but after the click on the arrow button of the combo box, nothing is displayed, no box with items for selection... What is wrong with my source code?