Flutter 参数类型Future>不能分配给参数类型Future>

我正在try 使用我的Cloud FiRestore数据库中的数据，但我收到了这个错误，我不知道为什么会出现这个错误，也不知道如何修复它.就我而言，错误所涉及的两种数据类型是相同的.

以下是错误: The argument type 'Future<List<Exercise>>' can't be assigned to the parameter type 'Future<List<Exercise>>?'个 以下是出现错误的代码片段.错误出在这一部分:FirestoreService().getExercises()

child: FutureBuilder<List<Exercise>>(
   future: FirestoreService().getExercises(),
   builder: (context, snapshot)

完整代码: exercise_list.dart个

class ExerciseList extends StatelessWidget {
  const ExerciseList({super.key});

  @override
  Widget build(BuildContext context) {
    return Expanded(
      child: DecoratedBox(
        decoration: const BoxDecoration(
          color: Colors.white,
        ),
        child: Padding(
          padding: const EdgeInsets.symmetric(vertical: 20, horizontal: 30),
          child: Column(
            crossAxisAlignment: CrossAxisAlignment.start,
            children: [
              Expanded(
                child: FutureBuilder<List<Exercise>>(
                  future: FirestoreService().getExercises(),
                  builder: (context, snapshot) {
                    if (snapshot.hasError) {
                      return const Text('Something went wrong');
                    }

                    if (snapshot.connectionState == ConnectionState.waiting) {
                      return const Center(
                        child: CircularProgressIndicator(),
                      );
                    }

                    return ListView.builder(
                      itemCount: snapshot.data!.length,
                      itemBuilder: (context, index) => ExerciseItem(
                        exercise: snapshot.data![index],
                      ),
                    );
                  },
                ),
              ),
            ],
          ),
        ),
      ),
    );
  }
}

firestore.dart个

/// Reads all documents from the exercises collection
  Future<List<Exercise>> getExercises() async {
    var ref = _db.collection('exercises');
    var snapshot = await ref.get();
    var data = snapshot.docs.map((s) => s.data());
    var exercises = data.map((d) => Exercise.fromJson(d));
    return exercises.toList();
  }

如有任何帮助，我们将不胜感激