问如何解决将Json转换为Dart数据模型的问题

EN

I/flutter (13849): type '(dynamic) => Null' is not a subtype of type '(String, dynamic) => void' of 'f'

print(value.folder.placemark[a].multiGeometry.polygonkml
                .innerBoundaryIs.linearRing[c].coordinates);

class InnerBoundaryIs {
  List<LinearRing> linearRing;

  InnerBoundaryIs({this.linearRing});

  InnerBoundaryIs.fromJson(Map<String, dynamic> json) {
    if (json['LinearRing'] != null) {
      linearRing = new List<LinearRing>();
      json['LinearRing'].forEach((v) {
        linearRing.add(new LinearRing.fromJson(v));
      });
    }
  }

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

class LinearRing {
  String tessellate;
  String coordinates;

  LinearRing({this.tessellate, this.coordinates});

  LinearRing.fromJson(Map<String, dynamic> json) {
    tessellate = json['tessellate'];
    coordinates = json['coordinates'];
  }

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