title: jQuery UI sortable({ stop:function(){} }) causing error - this.helper is null
body: <p>first I have a sortable container with <code>class="viewport"</code>, then I want show each child's id after be sorted, so the code:   </p>
  
  <pre><code>$(".viewport").sortable({
                      connectWith: '.viewport',
                      revert: true,
                      stop: function () {
                              $sort_left_array=$(this).children();
                              $sort_left_arry.each(function(){
                                  alert($(this).attr("id"));
                              });
                      }
                  });
  </code></pre>
  
  <p>but after sorted, in firebug, it alerts "this.helper is null", the alert function doesn't work, also the page's all animate don't work.</p>
  
  <p>by the way the items in sortable container is loaded with ajax  when the page is loading,in the <code>$(function(){})</code> part at the head of the page, does that matter?</p>
  
  <p>Has anyone had this problem before? How can I solve this problem?</p>