W / System.err：org.json.JSONException：org.json.JSONObject $ 1类型的值null无法转换为JSONObject

Question

这是我的代码

class BackgroundTask extends AsyncTask<Void, Void, String> {
    String target;

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        try {
            target = "http://sanghoboy135.cafe24.com/courseList.php?courseUniversity=" + URLEncoder.encode(courseUniversity, "UTF-8") +
                    "&courseYear=" + URLEncoder.encode(yearSpinner.getSelectedItem().toString().substring(0, 4), "UTF-8") + "&courseTerm=" + URLEncoder.encode(termSpinner.getSelectedItem().toString(), "UTF-8") +
                    "&courseArea=" + URLEncoder.encode(areaSpinner.getSelectedItem().toString(), "UTF-8") + "&courseMajor=" + URLEncoder.encode(majorSpinner.getSelectedItem().toString(), "UTF-8");

        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    @Override
    protected String doInBackground(Void... voids) {
        try {
            URL url = new URL(target);
            HttpURLConnection connection = (HttpURLConnection) url.openConnection();
            //if (connection != null) {
              //  connection.setRequestMethod("GET");
               // connection.getResponseCode();
            //}

            InputStream inputStream = connection.getInputStream();
            BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream));
            String temp = "";
            StringBuilder builder = new StringBuilder();

            while ((temp = reader.readLine()) != null) {
                builder.append(temp + "\n");
                Log.d("CourseFragment1", temp);
            }
            reader.close();
            inputStream.close();
            connection.disconnect();
            return builder.toString().trim();
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }

    @Override
    protected void onProgressUpdate(Void... values) {
        super.onProgressUpdate(values);
    }

    @Override
    protected void onPostExecute(String s) {
        super.onPostExecute(s);

        Log.d("CourseFragment2", s);
        try {
            String prefix = "null";
            if(s.startsWith(prefix)){
                s = s.substring(prefix.length(), s.length());
            }
            JSONObject jsonObject = new JSONObject(s);
            JSONArray jsonArray = jsonObject.getJSONArray("response");

            int count = 0;

            int courseID;
            String courseUniversity;
            int courseYear;
            String courseTerm;
            String courseArea;
            String courseMajor;
            String courseGrade;
            String courseTitle;
            int courseCredit;
            int courseDivide;
            int coursePersonnel;
            String courseProfessor;
            String courseTime;
            String courseRoom;

            courseList = new ArrayList<Course>();

                while (count < jsonArray.length()) {
                    JSONObject object = jsonArray.getJSONObject(count);

                    courseID = object.getInt("courseID");
                    courseUniversity = object.getString("courseUniversity");
                    courseYear = object.getInt("courseYear");
                    courseTerm = object.getString("courseTerm");
                    courseArea = object.getString("courseArea");
                    courseMajor = object.getString("courseMajor");
                    courseGrade = object.getString("courseGrade");
                    courseTitle = object.getString("courseTitle");
                    courseCredit = object.getInt("courseCredit");
                    courseDivide = object.getInt("courseDivide");
                    coursePersonnel = object.getInt("coursePersonnel");
                    courseProfessor = object.getString("courseProfessor");
                    courseTime = object.getString("courseTime");
                    courseRoom = object.getString("courseRoom");

                    Course course = new Course(courseID, courseUniversity, courseYear, courseTerm, courseArea, courseMajor, courseGrade, courseTitle, courseCredit, courseDivide, coursePersonnel, courseProfessor, courseTime, courseRoom);
                    courseList.add(course);


                    count++;
                }




            //Course course = new Course(1, "학부", 2016, "1학기", "교양및기타", "군사학", "3", "군사학", 2, 1, 0, "허진회", "", "");

            //courseList.add(course);
            adapter = new CourseAdapter(getActivity(), courseList);
            courseListView.setAdapter(adapter);
            /*if(count == 0){
                AlertDialog.Builder dialog = new AlertDialog.Builder(getActivity());
                dialog.setMessage("조회된 강의가 없습니다.")
                        .setPositiveButton("확인", null)
                        .create();
                dialog.show();
            }
            */
            //adapter.notifyDataSetChanged();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
  }
}

Logcat

D/CourseFragment1: NULL
                   NULL

D/CourseFragment1: {"response":[{"courseID":"1","courseUniversity":"학부","courseYear":"2016","courseTerm":"2학기","courseArea":"교양및기타","courseMajor":"군사학","courseGrade":"3","courseTitle":"군사학Ⅱ","courseCredit":"2","courseDivide":"1","coursePersonnel":"0","courseProfessor":"허진회","courseTime":"","courseRoom":""},{"courseID":"2","courseUniversity":"학부","courseYear":"2016","courseTerm":"2학기","courseArea":"교양및기타","courseMajor":"군사학","courseGrade":"4","courseTitle":"군사학Ⅳ","courseCredit":"2","courseDivide":"1","coursePersonnel":"0","courseProfessor":"허진회","courseTime":"","courseRoom":""}]}

错误

  

W / System.err：org.json.JSONException：类型的值null   org.json.JSONObject $ 1无法转换为JSONObject

我真的不知道为什么json get返回null和JSONArray。代码中是否有任何问题？还是应该只获取没有空值的JSONArray？