[bug report] Vue the notification component is used in the global customization instruction, and an exception pop-up window appears.

### Element UI version
2.13.2

### OS/Browsers version
chrome 版本 81.0.4044.138（正式版本） （64 位）

### Vue version
2.6.10

### Reproduction Link
https://codepen.io/jddk/pen/PoZmWWx

### Steps to reproduce
在登录页，f5刷新可以重现，右上角会出现。http://demo1.sdfymj.com:8086/#/login

### What is Expected?
无通知弹窗

### What is actually happening?
有无标题通知弹窗