addToScene method
- @override
override
    Override this method to upload this layer to the engine.
Return the engine layer for retained rendering. When there's no corresponding engine layer, null is returned.
Implementation
@override
ui.EngineLayer addToScene(ui.SceneBuilder builder, [Offset layerOffset = Offset.zero]) {
  addChildrenToScene(builder, layerOffset);
  return null; // ContainerLayer does not have a corresponding engine layer
}