Jetty embedded + Vaadin + Add-on

Hello.
I try to learn how to make own add-ons.
I try to use that add-on:
http://vaadin.com/directory#!addon/client-side-confirm-button

I was make these steps:

  1. Download sources from GITHub (
    https://github.com/daniel-sc/vaadin-client-side-confirm-button
    )
  2. Extract sources to classpath of existing project. Extract these files:
  • ConfirmButtonWidgetset.gwt.xml
  • ConfirmButton.java
    client/ConfirmButtonConnector.java
    client/ConfirmButtonState.java
  1. Add new component:

ConfirmButton button = new ConfirmButton("Save to click"); button.addClickListener(e -> System.out.println("Button was clicked and confirmed")); button.setCancelButtonIcon(FontAwesome.TIMES); button.setConfirmButtonIcon(FontAwesome.CHECK_CIRCLE); button.setConfirmQuestion("Are you really sure?"); button.setConfirmText("I am sure!"); button.setCancelText("I am not really sure.."); horizontalLayout.addComponent(button); 4. From Maven start “package”
5. Start application

In the result: i can see this Button, can click on it and view System.out.println() messages on Java console.
But i can’t get to show any confirmation elements.

I’m newby Vaadin-developer and may it be i loose some need steps?

Wait. I try to add this ConfirmButton over Maven and recompile project, but there is same problems.
Then i try to add over Maven another add-on
https://vaadin.com/directory#!addon/messagebox

It works well.
Then i try to disable Maven dependency and download sources of MessageBox.
After adding sources to project and recompile them it works well.
May it be that ConfirmButton contains bug?