@Test
public void whenEmptyUUIDIsGenerated () {

    when(internalStore.isEmpty()).thenReturn(true);
    when(internalStore.get()).thenReturn(Observable.just("uuid"));
    TestSubscriber<String> ts = new TestSubscriber<>();
    store.get().subscribe(ts);
    assertThat(ts.getOnNextEvents().size(), equalTo(1));
    verify(internalStore).store();
}