output: #fixed code
                                @Override
    public void handle(Message<JsonObject> message) {
      String action = getMandatoryString("action", message);
  
      if (action == null) {
        sendError(message, "No action specified.");
        return;
      }
  
      switch (action) {
        case "receive":
          doReceive(message);
          break;
        default:
          sendError(message, String.format("Invalid action %s.", action));
      }
    }
input: #vulnerable code 
                                @Override
    public void handle(Message<JsonObject> message) {
      String action = getMandatoryString("action", message);
      if (action == null) {
        sendError(message, "No action specified.");
      }
  
      switch (action) {
        case "receive":
          doReceive(message);
          break;
        default:
          sendError(message, String.format("Invalid action %s.", action));
      }
    } 
                              #location 8 
                              #vulnerability type NULL_DEREFERENCE