code: @Nullable
      public Controller getControllerWithInstanceId(@NonNull String instanceId) {
          for (RouterTransaction transaction : backstack) {
              Controller controllerWithId = transaction.controller.findController(instanceId);
              if (controllerWithId != null) {
                  return controllerWithId;
              }
          }
          return null;
      }
docs: Returns the hosted Controller with the given instance id or {@code null} if no such
  Controller exists in this Router.
  
  @param instanceId The instance ID being searched for