dynamic noSuchMethod(Invocation invocation) { throw new NoSuchMethodError.withInvocation(this, invocation);

I'm wondering if the given value is not null, but why is it giving an error null trying to enter ? or ! List<Cal>? KcalData; if changing from Cal to String

wrong in these two lines

KcalData.add(doc["foodName"]);          
KcalData.add(doc["kCal"].toString());

List<Cal>? KcalData; If Cal will get another error which I don't know how to fix it.
KcalData?.add(doc["foodName"]);
KcalData?.add(doc["kCal"]);
Which one should be fixed and there is no error?

class Cal{
  String? foodName;
  double? kCal;
  String? colorVal;
  Cal(this.foodName,this.kCal,this.colorVal);

This section is fetching data from firebase.

Widget bulidChart(BuildContext context) {
    return StreamBuilder<QuerySnapshot>(
        stream: FirebaseFirestore.instance.collection("chart").snapshots(),
        builder: (context, snapshot) {
          if (!snapshot.hasData) 
            return LinearProgressIndicator();
       
            return SizedBox(height:200,child: ListView(children: getExpenseItems(snapshot)));
          
        });
  }

getExpenseItems(AsyncSnapshot<QuerySnapshot> snapshot) {
    return snapshot.data!.docs
        .map((doc) {
                 
            KcalData?.add(doc["foodName"]); 
         
            KcalData?.add(doc["kCal"].toString());
                  
        })
        .toList();
  }
}

Widget _buildChart(BuildContext context,List<Cal> KcalData ) {
    List<charts.Series<Cal, String>>? datachart;
    _generateData(KcalData) {
    datachart!.add(
      charts.Series(
          id: 'Kcal',
          data: KcalData,
          labelAccessorFn: (Cal row, _) => "${row.foodName}",
          domainFn: (Cal _cal, _) => _cal.foodName.toString(),
          measureFn: (Cal _cal, _) => _cal.kCal,
          colorFn: (Cal _cal, _) =>
              charts.ColorUtil.fromDartColor(Color(int.parse(_cal.colorVal!)))),
    );
  }
    
    _generateData(KcalData);
    return Padding(
      padding: EdgeInsets.all(10),
      child: Container(
        child: Center(
            child: Column(
          children: [
            charts.PieChart(
              datachart!,
              animate: true,
              
              behaviors: [
                charts.DatumLegend(
                    entryTextStyle: charts.TextStyleSpec(
                        color: charts.MaterialPalette.purple.shadeDefault,
                        fontSize: 20,
                        fontFamily: 'Mitr'))
              ],
            )
          ],
        )),
      ),
    );
  }