[Bug Report] Can't open new message box in beforClose hooks function

### Element UI version
2.14.1

### OS/Browsers version
mac os 10.15.6 (19G2021) / google chrome 87.0.4280.67

### Vue version
3.0.4

### Reproduction Link
https://codepen.io/FortnightX/pen/XWjKXbd

### Steps to reproduce
1. click the "Click Me" button in page.（点击页面上的按钮）
2. click confirm button in the message box. （点击确认弹框上的确定按钮）
3. click the "Click Me" button in page again. （再次点击页面上的按钮）

### What is Expected?
The alert message box will appear immediately after the confirm box disappears.

alert 弹窗会在 confirm 弹窗消失后立刻出现。

### What is actually happening?
The alert message box will not appear after the confirm box disappears, but it appears first when you click Click Me for the second time.

alert 弹窗不会在 confirm 弹窗消失后立刻出现，而且会在下一次点击 Click Me 时首先弹出。