[Bug Report] When adding dropdown list to column, the dropdown items don't appear

### Element UI version
0.1.0

### OS/Browsers version
Mac Mojave 10.14.4 / Chrome Version 79.0.3945.88

### Vue version
3.0.0-alpha.1

### Reproduction Link
https://codepen.io/u-amine/pen/ZEYrjqa

### Steps to reproduce
Running the code at codepen already shows the bug.

### What is Expected?
The items from the should appear when the user clicks on the button.

### What is actually happening?
The items of the dropdown list are not showing when the user clicks on the button.