highValueMapper property Null safety

ChartIndexedValueMapper<num>? highValueMapper

Field in the data source, which is considered as high value for the data points.

Defaults to null

Widget build(BuildContext context) {
   return Container(
       child: SfCartesianChart(
           series: <RangeColumnSeries<SalesData, num>>[
               RangeColumnSeries<SalesData, num>(
                dataSource: <SalesData>[
                   SalesData(2005, 24, 16),
                   SalesData(2006, 22, 12),
                   SalesData(2007, 31, 18),
               highValueMapper: (SalesData data, _) => data.high,
class SalesData {
  SalesData(this.year, this.high, this.low);
  final num year;
  final num high;
  final num low;


final ChartIndexedValueMapper<num>? highValueMapper;