Undefined property LongAnswerResponse::$answer_feedback

We managed to fix this. In long_answer_classes.inc , replace this line

'#default_value' => $this->answer_feedback with

'#default_value' => isset($this->answer_feedback) ? $this->answer_feedback : '',

  public function getReportFormAnswerFeedback($showpoints = TRUE, $showfeedback = TRUE, $allow_scoring = FALSE) {
    if (quiz_access_to_score() && $allow_scoring) {
      return array(
        '#title' => t('Enter feedback'),
        '#type' => 'textarea',
        '#default_value' => isset($this->answer_feedback) ? $this->answer_feedback : '',
        '#attributes' => array('class' => array('quiz-report-score')),
      );
    }
    return FALSE;
  }