code: synchronized void setDefaultParent(final Actor defaultParent) {
      if (defaultParent != null && this.defaultParent != null) {
        throw new IllegalStateException("Default parent already exists.");
      }
  
      this.defaultParent = defaultParent;
    }
docs: Sets the {@code defaultParent} {@code Actor} as the default for this {@code World}. (INTERNAL ONLY)
  @param defaultParent the {@code Actor} to use as the default parent