@Test
public void peekState() {

  assertThat(routerNavigator.peekState()).isNull();
  routerNavigator.pushRetainedState(TestState.STATE_1, attachTransitionOne, detachTransitionOne);
  assertThat(routerNavigator.peekState()).isEqualTo(TestState.STATE_1);
  routerNavigator.popState();
  assertThat(routerNavigator.peekState()).isNull();
}