reactionsFromJson function Null safety
Implementation
Map<String, List<ParticipantId>> reactionsFromJson(Map<String, dynamic> json) => json.map((key, value) {
// ignore: avoid_as
final participants = (value is Map ? value.values.expand((e) => e) : value) as Iterable;
return MapEntry(key, participants.cast<String>().map(ParticipantId.fromJson).toList());
});