code: synchronized void setPublicRoot(final Stoppable publicRoot) {
      if (publicRoot != null && this.publicRoot != null) {
        throw new IllegalStateException("The public root already exists.");
      }
  
      this.publicRoot = publicRoot;
    }
docs: Sets the {@code PublicRootActor} instances as a {@code Stoppable}. (INTERNAL ONLY)
  @param privateRoot the {@code Stoppable} protocol backed by the {@code PrivateRootActor}