Null safety errors on model from quicktype

I’m trying to use a json model for my project. This is theresult I get from quicktype:

import 'dart:convert';

Hisselist hisselistFromJson(String str) => Hisselist.fromJson(json.decode(str));

String hisselistToJson(Hisselist data) => json.encode(data.toJson());

class Hisselist {
  Hisselist({
    required this.code,
    required this.data,
  });

  String code;
  List<Datum> data;

  factory Hisselist.fromJson(Map<String, dynamic> json) => Hisselist(
        code: json["code"],
        data: List<Datum>.from(json["data"].map((x) => Datum.fromJson(x))),
      );

  Map<String, dynamic> toJson() => {
        "code": code,
        "data": List<dynamic>.from(data.map((x) => x.toJson())),
      };
}

class Datum {
  Datum({
    required this.id,
    required this.kod,
    required this.ad,
    required this.tip,
  });

  int id;
  String kod;
  String ad;
  Tip tip;

  factory Datum.fromJson(Map<String, dynamic> json) => Datum(
        id: json["id"],
        kod: json["kod"],
        ad: json["ad"],
        tip: tipValues.map[json["tip"]],
      );

  Map<String, dynamic> toJson() => {
        "id": id,
        "kod": kod,
        "ad": ad,
        "tip": tipValues.reverse[tip],
      };
}

enum Tip { HISSE }

final tipValues = EnumValues({"Hisse": Tip.HISSE});

class EnumValues<T> {
  Map<String, T> map;
  Map<T, String> reverseMap;

  EnumValues(this.map);

  Map<T, String> get reverse {
    if (reverseMap == null) {
      reverseMap = map.map((k, v) => new MapEntry(v, k));
    }
    return reverseMap;
  }
}

I added "required at Datum class for null safety but I can not these 2 problems :

error: Non-nullable instance field ‘reverseMap’ must be initialized.
(not_initialized_non_nullable_instance_field at [myapp]
lib/models/apis/hisselist.dart:69)

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error: The argument type ‘Tip?’ can’t be assigned to the parameter
type ‘Tip’. (argument_type_not_assignable at [myapp]
lib/models/apis/hisselist.dart:48)

How can I fix this? thanks for your help

>Solution :

It is possible to get null from reading json, thats why it is showing error on tipValues.map[json["tip"]] using bang! is risky, you can provide default value on null case like

tip: tipValues.map[json["tip"]]?? Tip.HISSE,

Or make tip nullable

  Tip? tip;

Now on EnumValues all item expected to be initialized, you can use late to promise that you will assign data before reading it
late Map<T, String> reverseMap;

or better make it nullable

 Map<T, String>? reverseMap;

class EnumValues<T> {
  Map<String, T> map;
  Map<T, String>? reverseMap;

  EnumValues(this.map);

  Map<T, String> get reverse {
    return reverseMap ??= map.map((k, v) => MapEntry(v, k));
  }
}

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