@Override
  public Component getComponentByRef(int ref) {
    return getOptionalComponentByRef(ref)
      .orElseThrow(() -> new IllegalArgumentException(String.format("Component with ref '%s' can't be found", ref)));
  }
@Test
  public void getComponentByRef_throws_ISE_if_root_has_not_been_set() {
    assertThatThrownBy(() -> underTest.getComponentByRef(12))
      .isInstanceOf(IllegalStateException.class)
      .hasMessage("Holder has not been initialized yet");
  }