code: @Nullable
      final Controller findController(@NonNull String instanceId) {
          if (this.instanceId.equals(instanceId)) {
              return this;
          }
  
          for (Router router : childRouters) {
              Controller matchingChild = router.getControllerWithInstanceId(instanceId);
              if (matchingChild != null) {
                  return matchingChild;
              }
          }
          return null;
      }
docs: Returns the Controller with the given instance id or {@code null} if no such Controller
  exists. May return the Controller itself or a matching descendant
  
  @param instanceId The instance ID being searched for