제목 그대로 nodejs에 json형식으로 post보내고 그에대한 응답값을 받아오는걸 구현중입니다.!!!

//2020-08-29 04:19:53.003 20737-20737/com.example.application E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.application, PID: 20737
    java.lang.NullPointerException: Attempt to invoke virtual method 'int java.lang.String.hashCode()' on a null object reference
        at com.example.application.signupActivity$JSONTask.onPostExecute(signupActivity.java:139)
        at com.example.application.signupActivity$JSONTask.onPostExecute(signupActivity.java:61)
        at android.os.AsyncTask.finish(AsyncTask.java:755)
        at android.os.AsyncTask.access$900(AsyncTask.java:192)
        at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:772)
        at android.os.Handler.dispatchMessage(Handler.java:107)
        at android.os.Looper.loop(Looper.java:237)
        at android.app.ActivityThread.main(ActivityThread.java:8034)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:496)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1076)

이러한 오류가 계속 뜹니다ㅠㅠㅠ

public class JSONTask extends AsyncTask<String, String, String>{   //61
        @Override
        protected String doInBackground(String... urls){

            String txtemail = email.getText().toString();
            String txtusername = username.getText().toString();
            String txtnickname = nickname.getText().toString();
            String txtpassword = password.getText().toString();
            String txtpassword2 = password2.getText().toString();
            String txtmemnum = memnum.getText().toString();

            try {
                JSONObject jsonObject = new JSONObject();
                jsonObject.accumulate("id",txtemail);
                jsonObject.accumulate("pw",txtpassword);
                jsonObject.accumulate("pw2",txtpassword2);
                jsonObject.accumulate("nickName",txtnickname);
                jsonObject.accumulate("userName",txtusername);
                jsonObject.accumulate("memNum",txtmemnum);

                HttpURLConnection connection =null;
                BufferedReader reader = null;

                try {
                    URL url = new URL(urls[0]);
                    connection = (HttpURLConnection) url.openConnection();

                    connection.setRequestMethod("POST");
                    connection.setRequestProperty("Cache-Control","no-cache");
                    connection.setRequestProperty("Content-Type","application/json");
                    connection.setRequestProperty("Accept","application/json");
                    connection.setDoOutput(true);
                    connection.setDoInput(true);
                    connection.connect();

                    OutputStream outputStream = connection.getOutputStream();
                    BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(outputStream));
                    writer.write(jsonObject.toString());
                    writer.flush();
                    writer.close();

                    InputStream stream = connection.getInputStream();

                    JSONObject jsonObject1=new JSONObject(stream.toString());
                    Object res_State = jsonObject1.get("res_State");

                    return res_State.toString();
                } catch (IOException e){
                    e.printStackTrace();
                } finally {
                    if (connection!=null){
                        connection.disconnect();
                    }
                }
                try {
                    if (reader!=null){
                        reader.close();
                    }
                } catch (IOException e){
                    e.printStackTrace();
                }
            } catch (Exception e){
                e.printStackTrace();
            }
            return null;
        }
        @Override
        protected void onPostExecute(String result){
            super.onPostExecute(result);

            switch (result) {     //139
                case "id_is_invalid_form":
                    Toast.makeText(signupActivity.this, "올바른 이메일을 입력하세요", Toast.LENGTH_SHORT).show();
                    break;
                case "pw_if_invalid_form":
                    Toast.makeText(signupActivity.this, "올바른 비밀번호를 입력하세요", Toast.LENGTH_SHORT).show();
                    break;
                case "nickName_is_invalid_form":
                    Toast.makeText(signupActivity.this, "올바른 닉네임을 입력하세요", Toast.LENGTH_SHORT).show();
                    break;
                case "userName_is_invalid_form":
                    Toast.makeText(signupActivity.this, "올바른 이름을 입력하세요", Toast.LENGTH_SHORT).show();
                    break;
                case "pw_do_not_match":
                    Toast.makeText(signupActivity.this, "두 비밀번호가 일치하지 않습니다.", Toast.LENGTH_SHORT).show();
                    break;
                case "email_is_already_used":
                    Toast.makeText(signupActivity.this, "사용중인 이메일 입니다.", Toast.LENGTH_SHORT).show();
                    break;
                case "nickname_is_already_used":
                    Toast.makeText(signupActivity.this, "사용중인 닉네임 입니다.", Toast.LENGTH_SHORT).show();
                    break;
                case "success":
                    Toast.makeText(signupActivity.this, "회원가입 되었습니다.", Toast.LENGTH_SHORT).show();
                    Intent intent = new Intent(signupActivity.this, MainActivity.class);
                    startActivity(intent);
                    finish();
                    break;
                default:
                    Toast.makeText(signupActivity.this, "회원가입에 실패했습니다..", Toast.LENGTH_SHORT).show();
                    break;
            }
        }
    }

로그캣에 뜨는 61번과 139번은 코드에 함께 적어두었습니다! 조금만 도와주세요ㅠㅠㅠ