错误抖动NoSuchMethodError（NoSuchMethodError：在null上调用了getter'length'。Receiver：null

Question

    class AddProduct extends StatefulWidget {
      @override
      _AddProductState createState() => _AddProductState();
    }
    
    class _AddProductState extends State<AddProduct> {
      CategoryService _categoryService = CategoryService();
      BrandService _brandService = BrandService();
      GlobalKey<FormState> _formKey = GlobalKey<FormState>();
      TextEditingController prodoctNameController = TextEditingController();
      List<DocumentSnapshot> brands = <DocumentSnapshot>[];
      List<DocumentSnapshot> categories = <DocumentSnapshot>[];
      List<DropdownMenuItem<String>> categoriesDropDown =
          <DropdownMenuItem<String>>[];
      List<DropdownMenuItem<String>> brandsDropDown = <DropdownMenuItem<String>>[];
      String _currentCategory = "test";
      String _brandCategory;
      Color white = Colors.white;
      Color black = Colors.black;
      Color grey = Colors.grey;
      Color blue = Colors.blue;
    
      @override
      void initState() {
        _getCategories();
        //_getBrands();
        categoriesDropDown = getCategoriesDropDown();
        //_currentCategory = categoriesDropDown[0].value;
      }
    
      List<DropdownMenuItem<String>> getCategoriesDropDown() {
        List<DropdownMenuItem<String>> items = new List();
        for (DocumentSnapshot category in categories) {
          items.add(new DropdownMenuItem(
            child: Text(category['category']),
            value: category['category'].value ?? "",
          ));
        }
        return items;
      }
    
      Widget build(BuildContext context) {
        return Scaffold(
          appBar: AppBar(
            backgroundColor: white,
            leading: Icon(
              Icons.close,
              color: black,
            ),
            title: Text(
              "Añadir producto",
              style: TextStyle(color: black),
            ),
          ),
          body: Form(
            key: _formKey,
            child: ListView(
              children: <Widget>[
                Row(
                  children: <Widget>[
                    Expanded(
                      child: Padding(
                        padding: const EdgeInsets.all(8.0),
                        child: OutlineButton(
                          borderSide:
                              BorderSide(color: grey.withOpacity(0.8), width: 1.0),
                          onPressed: () {},
                          child: Padding(
                            padding: const EdgeInsets.fromLTRB(14, 40, 14, 40),
                            child: new Icon(
                              Icons.add,
                              color: grey,
                            ),
                          ),
                        ),
                      ),
                    ),
                    Expanded(
                      child: Padding(
                        padding: const EdgeInsets.all(8.0),
                        child: OutlineButton(
                          borderSide:
                              BorderSide(color: grey.withOpacity(0.8), width: 1.0),
                          onPressed: () {},
                          child: Padding(
                            padding: const EdgeInsets.fromLTRB(14, 40, 14, 40),
                            child: new Icon(
                              Icons.add,
                              color: grey,
                            ),
                          ),
                        ),
                      ),
                    ),
                    Expanded(
                      child: Padding(
                        padding: const EdgeInsets.all(8.0),
                        child: OutlineButton(
                          borderSide:
                              BorderSide(color: grey.withOpacity(0.8), width: 1.0),
                          onPressed: () {},
                          child: Padding(
                            padding: const EdgeInsets.fromLTRB(14, 40, 14, 40),
                            child: new Icon(
                              Icons.add,
                              color: grey,
                            ),
                          ),
                        ),
                      ),
                    )
                  ],
                ),
                Padding(
                  padding: const EdgeInsets.all(8.0),
                  child: Text(
                    'Indicar nombre del producto',
                    textAlign: TextAlign.center,
                    style: TextStyle(
                      color: blue,
                      fontSize: 12,
                    ),
                  ),
                ),
                Padding(
                  padding: const EdgeInsets.all(12.0),
                  child: TextFormField(
                    controller: prodoctNameController,
                    decoration: InputDecoration(hintText: "Añadir nombre producto"),
                    // ignore: missing_return
                    validator: (value) {
                      if (value.isEmpty ?? "") {
                        return 'Por favor, introduzca el nombre';
                      } else if (value.length > 10) {
                        return 'No se puede tener mas de 10 letras';
                      }
                    },
                  ),
                ),
                Center(
                  child: DropdownButton(
                      value: _currentCategory,
                      items: categoriesDropDown,
                      onChanged: changeSelectedCategory),
                )
              ],
            ),
          ),
        );
      }
    
      _getCategories() async {
        List<DocumentSnapshot> data = await _categoryService.getCategories();
        print(data.length ?? "");
        setState(() {
          categories = data;
        });
      }
    
      changeSelectedCategory(String selectedCategory) {
        setState(() => _currentCategory = selectedCategory);
      }

}

错误是这样的：

发生异常。 NoSuchMethodError（NoSuchMethodError：The getter'length'在null上被调用。接收方：null尝试调用： 长度）

我需要将此添加到Firebase。但是我不能因为这个错误。