anchor.diff:        return numSplitters * DefaultSplitter.splitterWidth;
       } else {
         var totalSize = 0.0;
  -      for (var splitter in widget.splitters) {
  +      for (var splitter in widget.splitters!) {
           totalSize += isHorizontal
               ? splitter.preferredSize.width
               : splitter.preferredSize.height;
  
positive.code: }
  
  class DefaultSplitter extends StatelessWidget {
    const DefaultSplitter({@required this.isHorizontal});
  
    static const double iconSize = 24.0;
    static const double splitterWidth = 12.0;
  
negative.code:   @override
    Widget build(BuildContext context) {
      return Transform.rotate(
        angle: isHorizontal ? degToRad(90.0) as double : degToRad(0.0) as double,
        child: Align(
          widthFactor: 0.5,
          heightFactor: 0.5,
  