MultiChildRenderObjectWidget({Key key, List<Widget> children: const [] })

Initializes fields for subclasses.

The children argument must not be null and must not contain any null objects.

Source 

MultiChildRenderObjectWidget({ Key key, this.children: const <Widget>[] })
  : super(key: key) {
  assert(children != null);
  assert(!children.any((Widget child) => child == null));
}