我该如何解决 getter 'length' 被调用为 null。接收者：空尝试调用：长度？

Question

我有一个屏幕显示主类别的子类别，我使用 hive 来保存和接收我的应用程序数据，但是当我使用未来的构建器返回指定的对象时，我遇到了标题错误， 我分享页面和我的课程请帮助我谢谢...................................................... ....................

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class _SubCategoriesScreenState extends State<SubCategoriesScreen> {
  int _flag=0;

  void initState(){
    super.initState();
    if(widget.categoryId!=null && widget.subCategoryImagePath!=null && widget.subCategoryName!=null){
      addSubCategory();
      }
    }

    void addSubCategory()async{
      SubCategoryModel subCategoryModel=SubCategoryModel(
          subCategoryId:widget.categoryId,
          subCategoryImagePath: widget.subCategoryImagePath,
          subCategoryName: widget.subCategoryName,
          categoryColor: Color((Random().nextDouble() * 0xFFFFFF).toInt()).withOpacity(0.5).value);
      if(categoryModels!=null){
        for (var categoryModel in categoryModels){
          if (categoryModel.categoryId == subCategoryModel.subCategoryId){
            categoryModel.subCategoryModels.insert(categoryModel.subCategoryModels.length-1,subCategoryModel);
            categoryModel=await hiveHelper.getCategoryModel(widget.categoryId);
            categoryModel.subCategoryModels.add(subCategoryModel);
            hiveHelper.addCategoryModel(categoryModel);
          }
        }
      }
    }



  @override
  Widget build(BuildContext context) {
    return SafeArea(
      child: FutureBuilder(
        future: HiveHelper().getCategoryModel(widget.categoryId),
        builder: (BuildContext context,AsyncSnapshot<CategoryModel> snapshot) {
          var _categoryModel=snapshot.data;
          Future.delayed( Duration(milliseconds: 900));
          return  Scaffold(
            appBar: AppBar(
              automaticallyImplyLeading: false,
              centerTitle: true,
              title: BorderedText(
                child:Text(
                  _categoryModel.categoryName,
                  style: TextStyle(
                      color: Color(0XFFFFFB00),
                      fontSize: 30,
                      fontFamily: "OpenSans"
                  ),
                ),
                strokeWidth: 5,
                strokeColor: Colors.black,
              ),
              elevation: 5,
              backgroundColor: Color(0xFFF2C3D4).withOpacity(1),
              leading: IconButton(
                icon: Icon(Icons.arrow_back),
                onPressed: (){
                  Navigator.pop(context);
                },
                iconSize: 40,
                color: Color(0xFFA2000B),
              ),
              actions: [
                CircleAvatar(
                  radius: 27,
                  backgroundColor: Colors.transparent,
                  backgroundImage: AssetImage("images/cuttedlogo.PNG"),
                )
              ],
            ),
            body:Container(
                  decoration: BoxDecoration(
                    image: DecorationImage(
                      image: AssetImage("images/logoBGopacity.png"),
                      fit: BoxFit.cover,
                    ),
                  ),
                  child: Column(
                    crossAxisAlignment: CrossAxisAlignment.stretch,
                    children: [
                      Expanded(
                        child: GridView.builder(
                            scrollDirection: Axis.vertical,
                            gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(crossAxisCount: 2),
                            itemCount:snapshot.data.subCategoryModels.length+1,
                            itemBuilder: (context,index){
                              if(_categoryModel.subCategoryModels.length==0){
                                return EmptyCard(where: "subCategoryScreen",categoryId: widget.categoryId,);
                              }
                              if(_flag==1){
                                return EmptyCard(where: "homeScreen",);
                              }
                              if(index==_categoryModel.subCategoryModels.length-1){
                                _flag=1;
                                SubCategoryModel subCategoryModel = snapshot.data.subCategoryModels[index];
                                return SubCategoryCard(subCategoryCardId:index,subCategoryId:subCategoryModel.subCategoryId,subcategoryName: subCategoryModel.subCategoryName,
                                  subCategoryImagePath:subCategoryModel.subCategoryImagePath,
                                  subCategoryCardColor: subCategoryModel.categoryColor,);
                              }
                              else{
                                SubCategoryModel subCategoryModel =  snapshot.data.subCategoryModels[index];
                                return SubCategoryCard(subCategoryCardId:index,subCategoryId:subCategoryModel.subCategoryId,subcategoryName: subCategoryModel.subCategoryName,
                                  subCategoryImagePath:subCategoryModel.subCategoryImagePath,
                                  subCategoryCardColor: subCategoryModel.categoryColor,);
                              }
                            }
                        ),
                      ),
                      Column(
                        mainAxisAlignment: MainAxisAlignment.end,
                        crossAxisAlignment: CrossAxisAlignment.stretch,
                        children: [
                          Padding(
                            padding: EdgeInsets.all(10),
                            child: Container(
                              decoration: BoxDecoration(
                                border: Border.all(style: BorderStyle.solid),
                                color: kColorTheme7,
                                borderRadius: BorderRadius.circular(40),
                              ),
                              child: TextButton(
                                onPressed: (){
                                  showModalBottomSheet(
                                    isDismissible: false,
                                    enableDrag: false,
                                    context: context,
                                    builder: (BuildContext context)=> AddMenuScreen(categoryId:widget.categoryId,buttonText: "Tarif Ekle", route:"subCategoryScreen"),
                                  );
                                },
                                child: BorderedText(
                                  strokeWidth: 5,
                                  strokeColor: Colors.black,
                                  child:Text("Tarif Ekle",style: TextStyle(
                                    color: Colors.white,
                                    fontFamily:'OpenSans',
                                    fontSize:30,
                                ),
                              ),
                            ),
                          ),
                        ),
                      ),
                    ],
                  )
                ],
              ),
            ),
          );
        },
      ),
    );
  }
}

part 'categoryModel.g.dart';

@HiveType(typeId : 0)
class CategoryModel extends HiveObject{
  CategoryModel(
      { this.categoryId,
        this.categoryImagePath,
        this.categoryName,
        this.categoryColor,
        this.subCategoryModels});

  @HiveField(0)
  final int categoryColor;

  @HiveField(1)
  List <SubCategoryModel> subCategoryModels=[];

  @HiveField(2)
  int categoryId;

  @HiveField(3)
  String categoryImagePath;

  @HiveField(4)
  String categoryName;

  @HiveField(5)
  SubCategoryModel subCategoryModel;
}

part 'subCategoryModel.g.dart';


@HiveType(typeId : 1)
class SubCategoryModel extends HiveObject{
  SubCategoryModel({
    this.subCategoryId,
    this.subCategoryImagePath,
    this.subCategoryName,
    this.categoryColor,
    this.recipeId,
    List<Ingredient>ingredients,
    this.recipePhotoDir,
    this.recordedVoiceDir,
    bool isCompeted});

  @HiveField(0)
  final int categoryColor;

  @HiveField(1)
  final double recipeId;

  @HiveField(2)
  bool isCompleted;

  @HiveField(3)
  int subCategoryId;

  @HiveField(4)
  String subCategoryImagePath;

  @HiveField(5)
  String subCategoryName;

  @HiveField(6)
  List <Ingredient> ingredients=[] ;

  @HiveField(7)
  String recipePhotoDir;

  @HiveField(8)
  String recordedVoiceDir;

}