fix json serialization nnbd

packages/stream_chat/lib/src/models/attachment.dart

@@ -49,12 +49,12 @@ class Attachment extends Equatable {
   /// Create a new instance from a json
   factory Attachment.fromJson(Map<String, dynamic> json) =>
       _$AttachmentFromJson(
-          Serialization.moveToExtraDataFromRoot(json, topLevelFields)!);
+          Serialization.moveToExtraDataFromRoot(json, topLevelFields));
 
   /// Create a new instance from a db data
   factory Attachment.fromData(Map<String, dynamic> json) =>
       _$AttachmentFromJson(Serialization.moveToExtraDataFromRoot(
-          json, topLevelFields + dbSpecificTopLevelFields)!);
+          json, topLevelFields + dbSpecificTopLevelFields));
 
   ///The attachment type based on the URL resource. This can be: audio,
   ///image or video