title: jQuery UI Sortable - Error: cannot call methods on sortable prior to initialization; attempted to call method 'serialize'
body: <p>How to .sortable( "serialize" ) only #sortable1? </p>
  
  <p>Every time on #sortable1 Update I get error:</p>
  
  <blockquote>
    <p>Error: cannot call methods on sortable prior to initialization; attempted to call method 'serialize'</p>
  </blockquote>
  
  <p>jQuery code</p>
  
  <pre><code>$( "#sortable1, #sortable2" ).sortable({
    connectWith: ".connectedSortable",
    update: function(event, ui){
      var sorted = $("#sortable1").sortable( "serialize" );
      console.log( sorted );
    }
  }).disableSelection();
  </code></pre>
  
  <p>HTML code</p>
  
  <pre><code>        &lt;div class="row"&gt;
              &lt;div class="col-sm-6"&gt;
                      &lt;div id="sortable1" class="connectedSortable"&gt;
                        &lt;div class="ui-state-default" id="item_1"&gt;Item 1&lt;/div&gt;
                        &lt;div class="ui-state-default" id="item_2"&gt;Item 2&lt;/div&gt;
                        &lt;div class="ui-state-default" id="item_3"&gt;Item 3&lt;/div&gt;
                        &lt;div class="ui-state-default" id="item_4"&gt;Item 4&lt;/div&gt;
                        &lt;div class="ui-state-default" id="item_5"&gt;Item 5&lt;/div&gt;
              &lt;/div&gt;
  
              &lt;div class="col-sm-6"&gt;
                  &lt;div id="sortable2" class="connectedSortable"&gt;
                    &lt;div class="ui-state-highlight" id="item_11"&gt;Item 1&lt;/div&gt;
                    &lt;div class="ui-state-highlight" id="item_22"&gt;Item 2&lt;/div&gt;
                    &lt;div class="ui-state-highlight" id="item_33"&gt;Item 3&lt;/div&gt;
                    &lt;div class="ui-state-highlight" id="item_44"&gt;Item 4&lt;/div&gt;
                    &lt;div class="ui-state-highlight" id="item_55"&gt;Item 5&lt;/div&gt;
                  &lt;/div&gt;
              &lt;/div&gt;
          &lt;/div&gt;
  </code></pre>