After the [bug report] El drawer pops up and closes, the mask layer does not disappear (: modal append to body = "false")

### Element UI version
2.15.1

### OS/Browsers version
Windows

### Vue version
2.6.12

### Reproduction Link
https://codepen.io/hzsrc/pen/Baprdoj

### Steps to reproduce
1、点击Button
2、点击弹出的按钮
3、关闭Drawer即可重现

### What is Expected?
遮罩层正确关闭

### What is actually happening?
遮罩层未关闭