el-dropdown在刚开始可以正常显示下拉框
但是关闭弹窗后我再次点开就无法显示下拉框了，该如何解决呀？
直接复制官网的示例就是这样
```html
<el-dropdown trigger="click">
<span class="el-dropdown-link">
下拉菜单
<i class="el-icon-arrow-down el-icon--right"></i>
</span>
<el-dropdown-menu slot="dropdown">
<el-dropdown-item icon="el-icon-plus">黄金糕</el-dropdown-item>
<el-dropdown-item icon="el-icon-circle-plus">狮子头</el-dropdown-item>
<el-dropdown-item icon="el-icon-circle-plus-outline">螺蛳粉</el-dropdown-item>
<el-dropdown-item icon="el-icon-check">双皮奶</el-dropdown-item>
<el-dropdown-item icon="el-icon-circle-check">蚵仔煎</el-dropdown-item>
</el-dropdown-menu>
</el-dropdown>
```
