Flutter电商APP登录报错：Null不是Iterable子类型（cart字段）

我正在开发一款电商APP，登录页面的设计逻辑是：用户输入凭证并点击登录按钮后，展示用户的id、name、email、password、address、type、token、cart信息。APP能正常启动，但登录时弹出错误：

Exception has occurred. _TypeError (type 'Null' is not a subtype of type 'Iterable')

错误出现在处理cart字段的代码位置，注释掉cart相关代码后，报错消失，能正常读取登录凭证。相关代码如下：

import 'dart:convert';

class User {
  final String id;
  final String name;
  final String email;
  final String password;
  final String address;
  final String type;
  final String token;
  final List<dynamic> cart;

  User({
    required this.id,
    required this.name,
    required this.email,
    required this.password,
    required this.address,
    required this.type,
    required this.token,
    required this.cart,
  });

  Map<String, dynamic> toMap() {
    return {
      'id': id,
      'name': name,
      'email': email,
      'password': password,
      'address': address,
      'type': type,
      'token': token,
      'cart': cart,
    };
  }

  factory User.fromMap(Map<String, dynamic> map) {
    return User(
      id: map['_id'] ?? '',
      name: map['name'] ?? '',
      email: map['email'] ?? '',
      password: map['password'] ?? '',
      address: map['address'] ?? '',
      type: map['type'] ?? '',
      token: map['token'] ?? '',
      cart: List<Map<String, dynamic>>.from(
        map['cart']?.map(
          (x) => Map<String, dynamic>.from(x), // 错误出现位置
        ),
      ),
    );
  }

  String toJson() => json.encode(toMap());

  factory User.fromJson(String source) => User.fromMap(json.decode(source));

  User copyWith({
    String? id,
    String? name,
    String? email,
    String? password,
    String? address,
    String? type,
    String? token,
    List<dynamic>? cart,
  }) {
    return User(
      id: id ?? this.id,
      name: name ?? this.name,
      email: email ?? this.email,
      password: password ?? this.password,
      address: address ?? this.address,
      type: type ?? this.type,
      token: token ?? this.token,
      cart: cart ?? this.cart,
    );
  }
}

问题出在User.fromMap工厂方法的cart字段处理逻辑：当接口返回的map['cart']为null时，map['cart']?.map()会返回null，而List.from()要求传入Iterable类型参数，无法接受null，因此触发类型转换错误。

给map['cart']设置默认值，当它为null时传入空列表[]，确保map()方法调用的对象始终是Iterable类型。修改cart字段的初始化代码即可：

import 'dart:convert';

class User {
  final String id;
  final String name;
  final String email;
  final String password;
  final String address;
  final String type;
  final String token;
  final List<dynamic> cart;

  User({
    required this.id,
    required this.name,
    required this.email,
    required this.password,
    required this.address,
    required this.type,
    required this.token,
    required this.cart,
  });

  Map<String, dynamic> toMap() {
    return {
      'id': id,
      'name': name,
      'email': email,
      'password': password,
      'address': address,
      'type': type,
      'token': token,
      'cart': cart,
    };
  }

  factory User.fromMap(Map<String, dynamic> map) {
    return User(
      id: map['_id'] ?? '',
      name: map['name'] ?? '',
      email: map['email'] ?? '',
      password: map['password'] ?? '',
      address: map['address'] ?? '',
      type: map['type'] ?? '',
      token: map['token'] ?? '',
      // 修改此处：给cart设置默认空列表，避免null传入List.from
      cart: List<Map<String, dynamic>>.from(
        (map['cart'] ?? []).map(
          (x) => Map<String, dynamic>.from(x),
        ),
      ),
    );
  }

  String toJson() => json.encode(toMap());

  factory User.fromJson(String source) => User.fromMap(json.decode(source));

  User copyWith({
    String? id,
    String? name,
    String? email,
    String? password,
    String? address,
    String? type,
    String? token,
    List<dynamic>? cart,
  }) {
    return User(
      id: id ?? this.id,
      name: name ?? this.name,
      email: email ?? this.email,
      password: password ?? this.password,
      address: address ?? this.address,
      type: type ?? this.type,
      token: token ?? this.token,
      cart: cart ?? this.cart,
    );
  }
}

内容的提问来源于stack exchange，提问作者Marvin Ekokobe