Angularjs 角型js&；用户界面可排序

我使用UI sortable创建了嵌套的可排序列表。该结构是父子结构。父列表可排序，子列表可排序。与此类似：

这把小提琴正是我的问题。可以将父对象拖动到子列表中

<ul ui-sortable="vm.sortableOptions" class="list-group ui-sortable">
       <li  class="list-group-item">
            <div class="row">
                <div class="col-xs-12">
                    <ul ui-sortable="vm.sortableOptions2" class="list-group>
                       <li class="list-group-item" >
                            <div class="dd-handle row">
                                <div class="col-xs-1 ng-binding">1 </div>
                                <div class="col-xs-3 ng-binding">code-0</div>
                                <div class="col-xs-7 ng-binding">item 0</div>
                            </div>
                        </li><li class="list-group-item">
                            <div class="dd-handle row">
                                <div class="col-xs-1 ng-binding">2 </div>
                                <div class="col-xs-3 ng-binding">code-1</div>
                                <div class="col-xs-7 ng-binding">item 1</div>
                            </div>
                        </li>
                    </ul>



                </div>
            </div>
        </li>
</ul>

Uncaught TypeError: Cannot read property 'ngModel' of null