message: children should always return a copy. add childrenRef to get reference for performance purpose
diff: @@ -122,7 +122,7 @@ export default class Storage {
  }
  if (el.isGroup) {
  - const children = (el as Group).children();
  + const children = (el as Group).childrenRef();
  for (let i = 0; i < children.length; i++) {
  const child = children[i];
  