json异常与php文件中的会话

After looking for an idea on the internet for a few hours without success, I'm writing my question here. I'm using a session to get a variable in an other php file. When I create the json in Android studio, I get an error. I tried some tests to see if the problem was the session and when I put my session code in comments, my code works well. I'm wondering if my android file needs to allow a session or anything else because all of my php files work fine.

My code is bellow :

• php file which posts the params (Login.php)

  if (isset($_POST['email']) && isset($_POST['password'])) {
      $email = $_POST['email'];
      $_SESSION['email'] = $email;
      $password = $_POST['password'];}
  

• php file which contains all the functions I need (it's the function which uses a session to get the email) (DB_Functions.php)

  <?php session_start();
  public function getIdByEmail() {
          $stmt = $this->conn->prepare("SELECT id FROM users WHERE email = ?");
          $stmt->bind_param("s",$_SESSION['email']);
          $result = $stmt->execute();
  
  
          if ($result) {
              $user = $stmt->get_result()->fetch_assoc();
              $stmt->close();
              return $user;
          } else {
              return NULL;
          }
         session_destroy();
      }
  ?>
  

• this function is called in an other php file to store the id

• the problem comes from here : (android file)

        String uid = jObj.getString("uid");
  
                      JSONObject candidate = jObj.getJSONObject("candidate");
                      int id_users_fk = candidate.getInt("id_users_fk");
                      String name = candidate.getString("name");
                      String firstname = candidate.getString("firstname");
                      String training = candidate.getString("training");
                      String areaActivity = candidate.getString("area_activity");
                      String type = candidate.getString("type");
                      String language1 = candidate.getString("language1");
                      String levelLanguage1 = candidate.getString("level_language1");
                      String language2 = candidate.getString("language2");
                      String levelLanguage2 = candidate.getString("level_language2");
                      String language3 = candidate.getString("language3");
                      String levelLanguage3 = candidate.getString("level_language3");
                      String skill = candidate.getString("skill");
                      String geolocation = candidate.getString("geolocation");
                      String created_at = candidate.getString("created_at");
                      db.addNewCandidate(id_users_fk, name, firstname, training, areaActivity, type,
                              language1, levelLanguage1, language2, levelLanguage2,
                              language3, levelLanguage3, skill, geolocation,
                              uid, created_at);
  

Error :

System.err: org.json.JSONException: Value null at id_users_fk of type org.json.JSONObject$1 cannot be converted to int