[Bug Report] The Select Component cannot hide the Popover Component（Select组件无法收起Popover组件）

### Element UI version
2.15.7

### OS/Browsers version
Windows 10 64 / Chrome 86

### Vue version
2.6.14

### Reproduction Link
https://codepen.io/microkof/pen/dyZyaqd

### Steps to reproduce
1. Copy the Select component and Popover component from the official website's examples.

2. Click to open the Popover component.

3. Click to open the Select component and you will find that Popover is not stowed.

### What is Expected?
Click to open the Select component and Popover will stow.

### What is actually happening?
Click to open the Select component and you will find that Popover is not stowed.