modelChatType ctype = modelChatType.fromJson(json.decode(data));
        String j ="join";

        print(ctype.message);            //OK

        if(ctype.command is String) {    //Ok goes in
            print("Thsi is a string");
        }
        if(ctype.command.toString() == "join"){ //Ok goes in
            print("dssd");
        }

       if(ctype.command == j){       ///Fails : Exception
        onJoin(data);
    }

@JsonSerializable()
class modelChatType extends Object with _$modelChatTypeSerializerMixin {
    String command;
    String source;

    modelChatType.fromEmpty()
    {
        this.command = null;
        this.source= null;
    }

    modelChatType(
            this.command,
            this.source,
            );

    factory modelChatType.fromJson(Map<String, dynamic> json) =>
            _$modelChatTypeFromJson(json);
}

if(ctype.command == j){       ///Fails : Exception
            onJoin(data);
        }

Exception :
Type 'String' is not a subtype of type 'modelChatType'
#0      ChatManager.onMessage (package:insta_gig/Managers/chatManager.dart:60:20)
#1      _RootZone.runUnaryGuarded (dart:async/zone.dart:1316:10)
#2      _BufferingStreamSubscription._sendData (dart:async/stream_impl.dart:330:11)
#3      _BufferingStreamSubscription._add (dart:async/stream_impl.dart:257:7)