extra_data Parsing

[obrunsmann]

I got an error parsing extra_data on an activity:

flutter: type 'Null' is not a subtype of type 'Object' in type cast
flutter: #0      _$EnrichedActivityFromJson.<anonymous closure>
#1      MapMixin.map (dart:collection/maps.dart:170:28)
#2      _$EnrichedActivityFromJson
package:stream_feed/…/models/enriched_activity.g.dart:27
#3      new EnrichedActivity.fromJson
package:stream_feed/…/models/enriched_activity.dart:66
#4      FlatFeed.getEnrichedActivities.<anonymous closure>
package:stream_feed/…/client/flat_feed.dart:133
#5      MappedListIterable.elementAt (dart:_internal/iterable.dart:413:31)
#6      ListIterator.moveNext (dart:_internal/iterable.dart:342:26)
#7      new _List._ofEfficientLengthIterable (dart:core-patch/array.dart:93:27)
#8      new _List.of (dart:core-patch/array.dart:55:20)
#9      new List.of (dart:core-patch/array_patch.dart:52:20)
#10     ListIterable.toList (dart:_internal/iterable.dart:213:44)
#11     FlatFeed.getEnrichedActivities
package:stream_feed/…/client/flat_feed.dart:134

Maybe it would be nice to also allow null values while it's a weak map either?