很奇怪的js问题，偶尔报null错误

placeHolder.remove();
                        placeHolder = null;
                        $(this).css("zIndex",zIndex);
                        $(this).css("position",position);
                        $(this).css({
                            top:"auto",
                            left:"auto"
                        });
                         
                        endIndex = container.children().index($(this));
                         
                        if(opt.update!=null && staIndex!=endIndex && endIndex != -1){
                           opt.update($(this),staIndex,endIndex); 
                        }
                        position = null;
                        dragging = null;  
                        isMoving = false;