[bug report] [dialog] after setting the destroy-on-close attribute, if you need to use the pop-up box that depends on Popper internally, the second call will fail

### Element UI version
2.13.2

### OS/Browsers version
Windows10 / Chrome 82

### Vue version
2.6.11

### Reproduction Link
https://codepen.io/linkontoask/pen/MWaRPEV

### Steps to reproduce
1，打开弹框，鼠标移入到按钮上，关闭弹框
2，再次打开弹框，鼠标移入到按钮上，会发现弹框的位置并不是预期的

阅读源码时发现`referenceElm`没有得到及时的更新，我怀疑是dialog的key导致的，因为设置`destroy-on-close`属性后，只改变了组件的key

### What is Expected?
在预期的位置显示弹框

### What is actually happening?
未正确的显示弹框