code: @NonNull
      public final Router getChildRouter(@NonNull ViewGroup container, @Nullable String tag) {
          //noinspection ConstantConditions
          return getChildRouter(container, tag, true);
      }
docs: Retrieves the child {@link Router} for the given container/tag combination. If no child router for
  this container exists yet, it will be created. Note that multiple routers should not exist
  in the same container unless a lot of care is taken to maintain order between them. Avoid using
  the same container unless you have a great reason to do so (ex: ViewPagers).
  
  @param container The ViewGroup that hosts the child Router
  @param tag The router's tag or {@code null} if none is needed