如何使用内置值创建嵌套对象

我想创建一个嵌套对象以请求发送给api。非常感谢帮助。

下面是嵌套的构建值类

abstract class BuiltUpdateProfileRequest
    implements
        Built<BuiltUpdateProfileRequest, BuiltUpdateProfileRequestBuilder> {
  // fields go here
  String get firstName;
  String get lastName;
  String get phoneNumber;
  @nullable
  ProfileBilling get billing;

  BuiltUpdateProfileRequest._();

  factory BuiltUpdateProfileRequest(
          [updates(BuiltUpdateProfileRequestBuilder b)]) =
      _$BuiltUpdateProfileRequest;

  static Serializer<BuiltUpdateProfileRequest> get serializer =>
      _$builtUpdateProfileRequestSerializer;
}

abstract class ProfileBilling
    implements Built<ProfileBilling, ProfileBillingBuilder> {
  // fields go here
  @nullable
  String get address1;
  @nullable
  String get address2;
  @nullable
  String get city;
  @nullable
  String get state;
  @nullable
  String get country;
  @nullable
  String get zip;
  ProfileBilling._();

  factory ProfileBilling([updates(ProfileBillingBuilder b)]) = _$ProfileBilling;

  static Serializer<ProfileBilling> get serializer =>
      _$profileBillingSerializer;
}

以下是请求对象，但在电话号码下开票时引发错误，指出无法将Profile Profile Billing类型的值分配给ProfileBillingBuilder类型的变量。

 final ProfileBilling profileBilling = ProfileBilling((b) => b
      ..address1 = ""
      ..address2 = ""
      ..city = ""
      ..state = ""
      ..country = ""
      ..zip = "");

 final BuiltUpdateProfileRequest builtUpdateProfileRequest =
        BuiltUpdateProfileRequest((b) => b
          ..firstName = firstName
          ..lastName = lastName
          ..phoneNumber = phoneNo
          ..billing = profileBilling);