@Override
  public Component getRoot() {
    checkInitialized();
    return this.root;
  }
@Test
  public void getRoot_throws_ISE_if_root_has_not_been_set_yet() {
    assertThatThrownBy(() -> underTest.getRoot())
      .isInstanceOf(IllegalStateException.class)
      .hasMessage("Holder has not been initialized yet");
  }