In the [bug report] drawer, it can be stated in the document that the number type can be used, but the error is reported after setting

### Element UI version
2.12.0

### OS/Browsers version
Google 77.0.3865.90

### Vue version
3.0.0-alpha.10

### Reproduction Link
https://codepen.io/wyujian/pen/OJVqEoP?editable=true

### Steps to reproduce
点击“点我打开按钮”，宽度没有变成900，

### What is Expected?
期待宽度变为900

### What is actually happening?
实际报错了，设置出错