我得到一个未处理的异常：类型'Null'不是类型'FutureOR的子类型

API需要太多的时间来加载，有时加载得非常快。我提供了下面的代码。

 Future<List<dynamic>> getSubjects() async {
    SharedPreferences prefs = await SharedPreferences.getInstance();
    var token = prefs.getString('token');
    var result = await http.get(
        Uri.parse("xxxxxxxxxxxx"),
        headers: {
          'Accept': 'application/json',
          'Content-Type': 'application/x-www-form-urlencoded',
          'Authorization': "Bearer $token",
        });
    return jsonDecode(result.body)['result'];
  }

未来建筑代码

Padding(
            padding: const EdgeInsets.all(18.0),
            child: FutureBuilder(
              future: getSubjects(),
              builder: (BuildContext context, AsyncSnapshot snapshot) {
                if (snapshot.hasData) {
                  return ListView.builder(
                    shrinkWrap: true,
                    itemCount: snapshot.data?.length,
                    itemBuilder: (BuildContext context, int index) {
                      return Padding(
                        padding: const EdgeInsets.all(5.0),
                        child: Container(
                          decoration: BoxDecoration(
                            image: const DecorationImage(
                              image: AssetImage('assets/bg.png'),
                              fit: BoxFit.cover,
                            ),
                            color: Colors.amber[50],
                            borderRadius: BorderRadius.circular(10.0),
                            boxShadow: const [
                              BoxShadow(
                                  color: Colors.grey,
                                  blurRadius: 2.0,
                                  offset: Offset(2.0, 2.0))
                            ],
                          ),
                          child: Padding(
                            padding: const EdgeInsets.all(8.0),
                            child: Column(
                              mainAxisAlignment: MainAxisAlignment.start,
                              crossAxisAlignment: CrossAxisAlignment.start,
                              children: [
                                ListTile(
                                  title: Text(
                                    snapshot.data[index]['subject_name']
                                        .toString(),
                                    style: const TextStyle(color: Colors.white),
                                  ),
                                  trailing: Row(
                                    mainAxisSize: MainAxisSize.min,
                                    children: [
                                      Icon((Icons.book),
                                          color: Colors.amber.shade900),
                                      const SizedBox(
                                        width: 10.00,
                                      ),
                                    ],
                                  ),
                                  onTap: () {
                                    Navigator.push(
                                      context,
                                      MaterialPageRoute(
                                        builder: (context) =>
                                            GiveFeedbackScreen(
                                          subjectId: snapshot.data[index]
                                              ['subject_id'],
                                          subjectName: snapshot.data[index]
                                              ['subject_name'],
                                          feedbackId: widget.id,
                                        ),
                                      ),
                                    );
                                  },
                                ),
                              ],
                            ),
                          ),
                        ),
                      );
                    },
                  );
                } else {
                  return Center(
                    child: LinearProgressIndicator(
                        backgroundColor: Colors.amber[500]),
                  );
                }
              },
            ),
          ),