toJson method Null safety

Implementation

@override
Map<String, dynamic> toJson() {
  final json = <String, dynamic>{};
  json['type'] = type;
  json.addAll({
    'traceEvents': traceEvents?.map((f) => f.toJson()).toList(),
    'timeOriginMicros': timeOriginMicros,
    'timeExtentMicros': timeExtentMicros,
  });
  return json;
}