未捕获的TypeError：无法读取属性＆lt; innerHTML＆＃39; null ajax jquery

Question

我试图用ajax做即时评论，我收到一个错误，说未捕获的TypeError：无法读取属性＆inner;内部HTML＆＃39; null我可以解决它，任何人都可以帮我解决这个问题吗？我正在按照教程（http://www.allaboutphp.com/php-ajax-live-comment-system/）

<script type="text/javascript">
        function sendPost() {
            var comment = document.getElementById("comment").value;
            var name = document.getElementById("username").value;
                if (comment && name) {
                    $.ajax
                        ({
                            type: 'POST',
                            url: 'post_comments.php',
                            data: {
                            user_comm: comment,
                            user_name: name
                            },
                            success: function (response) {
                            console.log(response);
                            document.getElementById("commentList").innerHTML = response + document.getElementById("commentList").innerHTML;
                            document.getElementById("comment").value = "";
                            document.getElementById("username").value = "";

                            }
                        });
                    }

            return false;
            }
    </script>

这是我的索引页面以显示评论....

                            <?php
                                $servername = "localhost";
                                $username = "root";
                                $password = "";

                                // Create connection
                                $con = new mysqli($servername, $username, $password);
                                $db = mysqli_select_db($con,"Review");
                                $qry = mysqli_query($con,"select name,comment,post_time from comments order by post_time desc");
                            while($row=mysqli_fetch_array($qry))
                            {
                              $name=$row['name'];
                              $comment=$row['comment'];
                              $time=$row['post_time'];
                        ?>


<div class="detailBox" id="detailBox">
    <div class="titleBox">
      <label>Comment Box</label>
        <button type="button" class="close" aria-hidden="true">&times;</button>
    </div>
    <div class="commentBox">

        <p class="taskDescription">test</p>
    </div>
    <div class="actionBox" id="actionBox">
        <ul class="commentList" id="commentList">
            <li>
                <div class="commenterImage">
                  <p><?php echo $name;?>:</p>
                </div>
                <div class="commentText">
                    <p class=""><?php echo $comment;?></p> <span class="date sub-text">on <?php echo $time;?></span>
                </div>

            </li>
        </ul>
        <?php
         }
        ?>
       <form class="form-inline" role="form" method="post" onsubmit="return sendPost();">
            <div class="row">
                <div class="col-md-12">
                    <div class="form-group">
                        <input class="form-control" type="text" placeholder="Your name" / required="" id="username">
                    </div>
                </div>
            </div>
            <div class="row">
                <div class="col-md-12">
                    <div class="form-group">
                        <textarea class="form-control" id="comment" placeholder="your comments....." required=""></textarea>
                    </div>
                </div>
            </div>
            <br>
            <div class="form-group">
                <input class="btn btn-default" type="submit" value="Submit">
            </div>
        </form>
    </div>
</div>