未处理的异常：NoSuchMethodError：在 null 上调用了方法“add”

Question

我有一个 Expanded 小部件，在这个小部件中，我创建了新的成分对象，然后我希望它添加另一个接受成分对象的类的列表，但是当我尝试使用列表的添加功能时，我遇到了错误如下。

Expanded(
                  flex: 40,
                  child: ListView.builder(
                    shrinkWrap: true,
                    itemCount: items.length,
                    itemBuilder: (context, index) {
                      return Padding(
                        padding: EdgeInsets.all(2.0),
                        child: GestureDetector(
                          onTap: ()async{
                            if(_ingredients.length==0){
                              Ingredient ingredient= Ingredient(ingredientName:"${items[index]}",dropDownValue: "Çay Kaşığı",ingredientAmount:null);
                              categoryModels[widget.subCategoryId].subCategoryModels[widget.subCategoryCardId].ingredients.add(ingredient);
                              addIngredient(ingredient,context);
                              _toggleCardHeight();
                              _toggleCardSize();
                              setState(() {

                              });
                            }
                            else{
                              var flag=0;
                              for(var i in _ingredients){
                                if(i.ingredientName==items[index]){
                                  flag=-1;
                                  var response= await showAlertDialog(context);
                                  if (response==false){
                                    setState(() {});
                                    }
                                  }
                                }
                              if(flag==0){
                                Ingredient ingredient=Ingredient(ingredientName:"${items[index]}",dropDownValue: "Çay Kaşığı",ingredientAmount: null);
                                categoryModels[widget.subCategoryId].subCategoryModels[widget.subCategoryCardId].ingredients.add(ingredient);
                                addIngredient(ingredient,context);
                                setState(() {});
                                }
                              }
                            },
                          child: Container(
                            decoration: BoxDecoration(
                              borderRadius: BorderRadius.circular(90),
                              border: Border.all(style:BorderStyle.solid,width: 1),
                              color: Colors.white54,

                            ),
                            child: Padding(
                              padding: EdgeInsets.all(5),
                              child: Text('${items[index]}',style: TextStyle(fontWeight: FontWeight.bold),)),
                          ),
                        ),
                      );
                    },
                  ),
                ),

这里是我想将成分作为对象添加到列表中的类。

class CategoryModel{
  CategoryModel(
    {
      this.categoryId,
      this.categoryImagePath,
      this.categoryName,
      this.categoryColor,
      this.subCategory
    });
  final SubCategoryModel subCategory;
  final Color categoryColor;
  final List<SubCategoryModel> subCategoryModels=[SubCategoryModel()];
  int categoryId;
  String categoryImagePath;
  String categoryName;

}
class SubCategoryModel{
  SubCategoryModel(
    {
      this.subCategoryId,
      this.subCategoryImagePath,
      this.subCategoryName,
      this.categoryColor,
      this.recipeId,
      this.ingredients,
      this.recipePhotoDir,
      this.recipeTextFromSpeechProcessing,
      this.recordedVoiceDir});

  final Color categoryColor;
  final double recipeId;
  int subCategoryId;
  String subCategoryImagePath;
  String subCategoryName;
  List<Ingredient> ingredients= [];
  String recipeTextFromSpeechProcessing;
  String recipePhotoDir;
  String recordedVoiceDir;
}

错误

E/flutter ( 7266): [ERROR:flutter/lib/ui/ui_dart_state.cc(186)] Unhandled Exception: NoSuchMethodError: The method 'add' was called on null.
E/flutter ( 7266): Receiver: null
E/flutter ( 7266): Tried calling: add(Instance of 'Ingredient')

有人可以帮我吗？我该如何处理这个问题。