Ng Bootstrap下拉菜单添加子下拉菜单不起作用

Question

我为 ng bootstrap drop down menu 添加了我的angular 7项目到子下拉菜单

我尝试使用以下方法，但不适用于我，任何人都知道如何正确添加此方法 谢谢

component.html                   

            <span  class="act-action text-info"  id="dropdownBasic1" ngbDropdownToggle>Action</span>
              <div ngbDropdownMenu aria-labelledby="dropdownBasic1">

                <div ngbDropdown class="d-inline-block">
                <a class="dropdown-item text-info"  style="cursor: pointer" id="dropdownBasic11" ngbDropdownToggle>Change Role</a>
                  <div ngbDropdownMenu aria-labelledby="dropdownBasic11">
                    <a class="dropdown-item" >Resend Email</a>
                    <a class="dropdown-item" >Cancel Invite</a>
                  </div>
                </div>
                <hr>
                <a class="dropdown-item" >Owner</a>
                <a class="dropdown-item" >Member</a>
                <a class="dropdown-item" >Member</a>
              </div>

          </div>


          </td>