GwtMockitoSetup.java

    private SampleWidget sampleWidget; // do not initialize here, but at the end of the setUp() method...
    private SimpleEventBus = new SimpleEventBus();

    @Before
    public void setUp() {
        GwtMockito.useProviderForType(SimpleEventBus.class, new FakeProvider() {
            @Override
            public Object getFake(Class aClass) {
                return simpleEventBus;
            }
        });

        sampleWidget = new SampleWidget();
    }