xValueMapper property Null safety

ChartIndexedValueMapper<D>? xValueMapper
final, inherited

Maps the field name, which will be considered as x-values.

Defaults to null

Widget build(BuildContext context) {
   return Container(
       child: SfFunnelChart(
           series: FunnelSeries<ChartData, String>(
               dataSource: <ChartData>[
                  ChartData('USA', 10),
                  ChartData('China', 11),
                  ChartData('Russia', 9),
                  ChartData('Germany', 10),
               xValueMapper: (ChartData data, _) => data.xVal,
class ChartData {
  ChartData(this.xVal, this.yVal);
  final String xVal;
  final int yVal;


final ChartIndexedValueMapper<D>? xValueMapper;