code: @Nullable
      public Controller getControllerWithTag(@NonNull String tag) {
          for (RouterTransaction transaction : backstack) {
              if (tag.equals(transaction.tag())) {
                  return transaction.controller;
              }
          }
          return null;
      }
docs: Returns the hosted Controller that was pushed with the given tag or {@code null} if no
  such Controller exists in this Router.
  
  @param tag The tag being searched for