反序列化来自cloud_firestore数据库的一个非常复杂的JSON对象时遇到一些问题。我已经采取了一种相当漂亮的方式格式化该对象。所以这是它的样子：

{
 "mealType": "Breakfast","id": 6,"totalCalories": 187.0,"nutritionixData": [
  {
   "phospate": 152.15,"totalFat": 11.11,"potassium": 173.4,"servingQuantity": 3.0,"photo": {
    "thumbnail": "https://d2xdmhkmkbyw75.cloudfront.net/9_thumb.jpg","highRes": null
   },"totalCarbohydrate": 0.04,"calories": 187.0,"nutrients": [
    {
     "attributeID": 203,"value": 20.366
    },{
     "attributeID": 204,"value": 11.1095
    },{
     "attributeID": 205,"value": 0.0425
    },{
     "attributeID": 207,"value": 0.7225
    },{
     "attributeID": 208,"value": 187.0
    },{
     "attributeID": 255,"value": 52.9975
    },{
     "attributeID": 268,"value": 782.0
    },{
     "attributeID": 291,"value": 0.0
    },{
     "attributeID": 301,"value": 11.05
    },{
     "attributeID": 303,"value": 1.3685
    },{
     "attributeID": 304,"value": 17.0
    },{
     "attributeID": 305,"value": 152.15
    },{
     "attributeID": 306,"value": 173.4
    },{
     "attributeID": 307,"value": 60.35
    },{
     "attributeID": 309,"value": 1.4535
    },{
     "attributeID": 312,"value": 0.0604
    },{
     "attributeID": 315,"value": 0.023
    },{
     "attributeID": 317,"value": 13.345
    },{
     "attributeID": 318,"value": 507.45
    },{
     "attributeID": 319,{
     "attributeID": 320,{
     "attributeID": 401,"value": 0.34
    },{
     "attributeID": 404,"value": 0.0485
    },{
     "attributeID": 405,"value": 0.1607
    },{
     "attributeID": 406,"value": 5.9764
    },{
     "attributeID": 410,"value": 0.8526
    },{
     "attributeID": 415,"value": 0.289
    },{
     "attributeID": 417,"value": 19.55
    },{
     "attributeID": 418,"value": 0.663
    },{
     "attributeID": 431,{
     "attributeID": 432,{
     "attributeID": 435,{
     "attributeID": 501,"value": 0.2253
    },{
     "attributeID": 502,"value": 0.8449
    },{
     "attributeID": 503,"value": 1.009
    },{
     "attributeID": 504,"value": 1.4841
    },{
     "attributeID": 505,"value": 1.6363
    },{
     "attributeID": 506,"value": 0.5372
    },{
     "attributeID": 507,"value": 0.2729
    },{
     "attributeID": 508,"value": 0.7973
    },{
     "attributeID": 509,"value": 0.652
    },{
     "attributeID": 510,"value": 0.9911
    },{
     "attributeID": 511,"value": 1.2895
    },{
     "attributeID": 512,"value": 0.5848
    },{
     "attributeID": 513,"value": 1.1815
    },{
     "attributeID": 514,"value": 1.8224
    },{
     "attributeID": 515,"value": 2.9801
    },{
     "attributeID": 516,"value": 1.3413
    },{
     "attributeID": 517,"value": 1.0098
    },{
     "attributeID": 518,"value": 0.7327
    },{
     "attributeID": 601,"value": 79.9
    },{
     "attributeID": 606,"value": 3.111
    },{
     "attributeID": 610,{
     "attributeID": 611,"value": 0.017
    },{
     "attributeID": 612,"value": 0.085
    },{
     "attributeID": 613,"value": 2.278
    },{
     "attributeID": 614,"value": 0.629
    },{
     "attributeID": 617,"value": 3.655
    },{
     "attributeID": 618,"value": 2.108
    },{
     "attributeID": 619,"value": 0.0935
    },{
     "attributeID": 620,"value": 0.102
    },{
     "attributeID": 621,"value": 0.0255
    },{
     "attributeID": 626,"value": 0.612
    },{
     "attributeID": 628,"value": 0.1105
    },{
     "attributeID": 629,"value": 0.0085
    },{
     "attributeID": 630,{
     "attributeID": 631,{
     "attributeID": 645,"value": 4.437
    },{
     "attributeID": 646,"value": 2.4225
    }
   ],"saturatedFat": 3.11,"serving_weight": 85.0,"sodium": 60.35,"foodName": "chicken","dietaryFiber": 0.0,"protein": 20.37,"cholesterol": 79.9,"sugar": 0.0
  }
 ],"meals": "chicken"
}

这是我尝试反序列化此对象的方法：

class FirebaseFood {
  int id;
  String mealType;
  String meals;
  List<FireNutritionix> fireNutritionix;
  double totalCalories;

  FirebaseFood(
      {this.fireNutritionix,this.id,this.meals,this.mealType,this.totalCalories});

  factory FirebaseFood.fromFirebase(Map<dynamic,dynamic> parsedFirebase) {
    var list = parsedFirebase['nutritionixData'] as List;
    List<FireNutritionix> fireNutritionix =
        list.map((i) => FireNutritionix.fromFirebase(i)).toList();

    return FirebaseFood(
        fireNutritionix: fireNutritionix,id: parsedFirebase['id'],meals: parsedFirebase['meals'],mealType: parsedFirebase['mealType'],totalCalories: parsedFirebase['totalCalories']);
  }
}

class FireNutritionix {
  double calories;
  double cholesterol;
  double dietaryFiber;
  List<FireNutrients> fireNutrients;
  double phospate;
  List<FirePhoto> photo;
  double potassium;
  double protein;
  double saturatedFat;
  double servingQuantity;
  double serving_weight;
  double sodium;
  double sugar;
  double totalCarbohydrate;
  double totalFat;

  FireNutritionix(
      {this.calories,this.cholesterol,this.dietaryFiber,this.fireNutrients,this.phospate,this.photo,this.potassium,this.protein,this.saturatedFat,this.serving_weight,this.servingQuantity,this.sodium,this.sugar,this.totalCarbohydrate,this.totalFat});

  factory FireNutritionix.fromFirebase(Map<dynamic,dynamic> parsedFirebase) {
      var list = parsedFirebase['nutrients'] as List; // being called on null
      List<FireNutrients> fireNutrients =
          list.map((i) => FireNutrients.fromFirebase(i)).toList();

      var list2 = parsedFirebase['photo'] as List; // being called on null
      List<FirePhoto> firePhoto = list2.map((i) => FirePhoto.fromFirebase(i));

    return FireNutritionix(
        calories: parsedFirebase['calories'],cholesterol: parsedFirebase['cholesterol'],dietaryFiber: parsedFirebase['dietaryFiber'],fireNutrients: fireNutrients,phospate: parsedFirebase['phospate'],photo: firePhoto,potassium: parsedFirebase['potassium'],protein: parsedFirebase['protein'],saturatedFat: parsedFirebase['saturatedFat'],serving_weight: parsedFirebase['serving_weight'],servingQuantity: parsedFirebase['servingQuantity'],sodium: parsedFirebase['sodium'],sugar: parsedFirebase['sugar'],totalCarbohydrate: parsedFirebase['totalCarbohydrate'],totalFat: parsedFirebase['totalFat']);
  }
}

class FireNutrients {
  int attributeID;
  double value;

  FireNutrients({this.attributeID,this.value});

  factory FireNutrients.fromFirebase(Map<dynamic,dynamic> parsedFirebase) {
    return FireNutrients(
        attributeID: parsedFirebase['attributeID'],value: parsedFirebase['value']);
  }
}

class FirePhoto {
  String highRes;
  String thumbnail;

  FirePhoto({this.highRes,this.thumbnail});

  factory FirePhoto.fromFirebase(Map<dynamic,dynamic> parsedFirebase) {
    return FirePhoto(
        highRes: parsedFirebase['highRes'],thumbnail: parsedFirebase['thumbnail']);
  }
}

我得到的错误是：

E/flutter ( 9185): Receiver: null
E/flutter ( 9185): Tried calling: map<FireNutrients>(Closure: (dynamic) => FireNutrients)
E/flutter ( 9185): #0      Object.noSuchMethod  (dart:core-patch/object_patch.dart:51:5)
E/flutter ( 9185): #1      new FireNutritionix.fromFirebase 
package:thesis/fromFirebase/firebaseFood.dart:69
E/flutter ( 9185): #2      _fitnessAppHomeScreenState.checkMealsList.<anonymous closure>.<anonymous closure> 
package:thesis/fitnessAppHomeScreen.dart:182
E/flutter ( 9185): #3      List.forEach  (dart:core-patch/growable_array.dart:283:8)

虽然我不明白为什么要对null调用parsedFirebase ['Nutritions']。.任何帮助和解释将不胜感激！非常感谢！