我最近迁移了我的应用程序以支持空安全。我使用provider包来管理我的应用程序状态。即使我检查了可空性，我仍然会在代码中遇到错误。下面是错误堆栈：
flutter：对空值使用了空检查运算符[VERBOSE-2：dart_vm_initializer.cc（41）] RangeError（index）：无效值：有效值范围为空：0 #0 List.[]（dart：core-patch/growable_array.dart：264：36）#1 Tutorials.setTutorials tutorials.dart：48 #2 _TutorialPageState.build. tutorial_page.dart：54 #3 Consumer.buildWithChild consumer.dart：179 #4 SingleChildStatelessWidget.build nested.dart：259 #5 StatelessElement.build framework.dart：5156 #6 SingleChildStatelessElement.build nested.dart：279 #7 ComponentElement.performRebuild framework.dart：5086 #8 Element.rebuild framework.dart：4805 #9 ComponentElement._firstBuild framework.dart：5068
下面是我的代码：
tutorials.dart

List<Tutorial>? _items = [];
  List<Tutorial>? get items {
    return [...?_items];
  }

  List<Tutorial>? get tutsOfASection {
    return [...?_tutsOfASection];
  }

  Tutorial? findById(int id) {
    return _tutsOfASection?.firstWhere((tut) => tut.id == id);
  }   

  setTutorials(int sectionId) async {
    final List<Tutorial>? loadedTuts = [];
    var i = 0;
    debugPrint("items: ${_items?[i].pid}");
    if (_items?[i] == null) {
      return;
    } else {
      do {
        if (_items![i].pid == sectionId) {
          loadedTuts?.add(_items![i]);
        }
        i++;
      } while (_items!.length > i);
      _tutsOfASection = loadedTuts;
    }
  }

字符串
tutorial_page.dart

body: Consumer<Tutorials>(builder: (context, value, child) {
    if (_section?["sectionId"] != null) {
      value.setTutorials(_section!["sectionId"]);
      return TutorialScreen();
    } else
      return Container();
  }),

型