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
3. 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?