[Bug Report] The first run cannot expand the child nodes of the first row; any other node can

### Element UI version
2.13.0

### OS/Browsers version
gogle

### Vue version
3.0.0-alpha.0

### Reproduction Link
https://raw.githubusercontent.com/Macile/UI-Automation-Trampling-Chapter/master/Tree_data%20-%20%E5%89%AF%E6%9C%AC.html

### Steps to reproduce
The first run cannot expand the child nodes of the first row; any other node can
第一次运行无法展开第一行的子节点，其他节点都可以

When the page is first opened, clicking the expand child button in the first row will not take effect, and there will be no response, no expansion, and no error. The other nodes are normal
在第一打开这个网页的时候，点击第一行的展开子节点按钮不会生效，没有任何反应，不会展开，也不会报错，其他节点正常

### What is Expected?
第一次打开网页第一行可以在点击展开的时候正常运行

The first line will work when you click expand

### What is actually happening?
第一次打开网页无法展开第一行的子节点

The first time you open a web page, you cannot expand the child node of the first row