bootstrap tab页面报错

刚开始页面加载的时候不报错，我依次点击1,2,3,4也没错，但是当我后退的时候点击3,2,1控制台就报什么函数未定义？这什么情况

 <!-- Nav tabs -->
<ul class="nav nav-tabs" role="tablist" id="maintab">
  <li class="active"><a href="#tab1"  onclick='showPage("tab1","trainee-info.htm")'>ABOUT</a></li>
  <li><a href="#tab2" onclick='showPage("tab2","sfunction-calendar.htm")'>#springMessage("sub-menu-tfunction-calendar")</a></li>
  <li><a href="#tab3" onclick='showPage("tab3","sfunction-my-course.htm")'>#springMessage("sub-menu-sfunction-my-course")</a></li>
  <li><a href="#tab4" onclick='showPage("tab4","my-team.htm")'>#springMessage("sub-menu-my-team")</a></li>
</ul>

<!-- Tab panes -->
<div class="tab-content">
    <div class="tab-pane active" id="tab1"></div>
    <div class="tab-pane" id="tab2"></div>
    <div class="tab-pane" id="tab3"></div>
    <div class="tab-pane" id="tab4"></div>
</div>



<script language="javascript">
    var loadimg="/img/tab-loding.gif"; // 加载时的loading图片
    $(function(){
        showPage("tab1","trainee-info.htm");
    })
    function showPage(tabId, url){
        $('#maintab a[href="#'+tabId+'"]').tab('show'); // 显示点击的tab页面
        if($('#'+tabId).html().length<20){ 
            $('#'+tabId).html('<br> <img src='+loadimg+' /> #springMessage('page-loading')'); 
            $('#'+tabId).load(url); // ajax加载页面
        }
    }

</script>