player_online.dart

void main() {
  var data =     {
    "data": [
        {
            "Name": "PlayerOne",
            "Data": "{\"isOnline\":false}"
        },
        {
            "Name": "PlayerTwo",
            "Data": "{\"isOnline\":false}"
        }
    ]
};
  PlayersOnline player = PlayersOnline.fromJson(data);
  
  print(player.data?.first.name);
}

class PlayersOnline {
  List<Data>? data;

  PlayersOnline({this.data});

  PlayersOnline.fromJson(Map<String, dynamic> json){
    data = List.from(json['data']).map((e)=>Data.fromJson(e)).toList();
  }

  Map<String, dynamic> toJson() {
    final Map<String, dynamic> data = new Map<String, dynamic>();
    if (this.data != null) {
      data['data'] = this.data?.map((v) => v.toJson()).toList();
    }
    return data;
  }
}

class Data {
  String? name;
  String? data;

  Data({this.name, this.data});

  Data.fromJson(Map<String, dynamic> json) {
    name = json['Name'];
    data = json['Data'];
  }

  Map<String, dynamic> toJson() {
    final Map<String, dynamic> data = new Map<String, dynamic>();
    data['Name'] = this.name;
    data['Data'] = this.data;
    return data;
  }
}