1. override
void detach()

Mark this node as detached.

Typically called only from the parent's detach, and by the owner to mark the root of a tree as detached.

Subclasses with children should detach all their children whenever this method is called.

Source 

@override
void detach() {
  if (_rowDecorationPainters != null) {
    for (BoxPainter painter in _rowDecorationPainters)
      painter?.dispose();
    _rowDecorationPainters = null;
  }
  for (RenderBox child in _children)
    child?.detach();
  super.detach();
}