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Flutter Struck with this snapshot Error Please help me to solve.

StreamBuilder(
        stream: _database.orderByKey().onValue
        builder: (context, snapshot) {
          final valudisp = <Card>[];
          if (snapshot.hasData) {
            final firstdata = new Map<String, dynamic>.from(
                (snapshot.data! as Event).snapshot.value);
            firstdata.forEach((key, value) {
              final _datalast = new Map<String, dynamic>.from(value);
              final _Qunatity =