Why this.helper = null ? (sortable / draggable / droppable)

To prevent a sort after a drop to the trash can, I'm using

$("#image-ul").sortable('cancel');

but getting an error in firebug. Everything works as expected, but paranoid that I've done something wrong. What way should I have stopped the sortable update after a droppable drop开发者_如何学C to the trash?

$(document).ready(function(){
    $(function(){
        // make the list of images sortable
        $("#image-ul").sortable({
            // allow dragging into the trash can
            connectWith: '#trash_can',
            item: 'li',
            // revert causes bugginess in IE, so left out even though cool
            //revert: 200,
            opacity: 0.6,
            cursor: 'move',
            // apply this css class to available places to drop within list
            placeholder: 'placeholder-border',
            // drop available once mouse has touched droppable area
            tolerance: 'pointer',
            update: function(){
                var order = $(this).sortable("serialize"); 
                $.post("/drag_drop/update", order, function(theResponse){
                    // show the confirmation and fade it away after 2.5 seconds
                    $('#content').css('display', 'block');
                    $('#content').html(theResponse).delay(2500).fadeOut('slow');
                });
            }
        });
        // allow for deleting images by sending them to the trash can
        $("#trash_can").droppable({
            accept: '#image-ul > li',
            // when the trash can is hovered on by a draggable, set the css class
            hoverClass: 'delete-border',
            drop: function(event, ui) {
                // setTimeout takes care of IE bug where deleted item remains
                setTimeout(function() { ui.draggable.remove(); }, 1);
                deleteImage(ui.draggable);
             }
        });
        // what to do when an image is dropped into the trash can
        function deleteImage($draggable){
            // strip out "image_data_" so that only the actual image ID is sent to delete query
            var id = $draggable.attr('id');
            id = id.replace('image_data_','');
            params = {
                    'id': id,
                    'path': $draggable.find("img").attr("src")
            }
            $.ajax({
                url: '/drag_drop/delete',
                type: 'POST',
                data: params,
                success: function( delete_response ){
                    $('#content').css('display', 'block');
                    $('#content').html( delete_response ).delay(2500).fadeOut('slow');
                }
            });
            $("#image-ul").sortable('cancel');
        }
    });
});

try instead:

$("#image-ul").sortable('disable');