Communcation error- inval Json response from server

Vaadin 6 / 7 / 8
1

I’m new to Vaadin and I’m trying to make an application that can be deployed to the google app engine.
So far I did 2 things 1) I created a Vaadin application in Maven that worked locally with Jetty 2) I created an ivy project as described in this tutorial (https://vaadin.com/forum#!/thread/9064078) and could deploy the template to the app engine.
Now when I tried to import my source file of 1 in 2 I got the following error when trying my application (the GUI is showing but the functionality is gone) communication problem - invalid JSON response from server. I also have the following errors in my console.

sep 14, 2016 7:32:07 AM com.vaadin.server.DefaultErrorHandler doDefault SEVERE: java.lang.NullPointerExceptionThis is the source of my UI implementation This is my source code of the UI

@SuppressWarnings("serial")
@Theme("games")
public class GamesUI extends UI {

    @Override
    protected void init(VaadinRequest request) {
        final VerticalLayout layout = new VerticalLayout();
        layout.setMargin(true);
        layout.addComponent(new InputForm(this));
        //layout.addComponent(new Button("click me"));
        setContent(layout);

            }

    @WebServlet(urlPatterns = "/*", name = "MyUIServlet", asyncSupported = true)
    @VaadinServletConfiguration(ui = GamesUI.class, productionMode = false)
    public static class MyUIServlet extends GAEVaadinServlet {
    }
}

anyone who can help to solve this problem?

Thanks