[Bug Report] 你好，element 2.15版本，自定义主题页面，获取配置和下载主题配置报错
Open
- Dominant language
- Vue
- Stars
- 54k
- Forks
- 14.4k
- PR merge metrics
- No merged PRs in 30d
Description
### Element UI version
2.15.14
### OS/Browsers version
*
### Vue version
2.7.16
### Reproduction Link
https://jsfiddle.net/mmx38qxw2/
### Steps to reproduce
点击编辑自定义主题，报错
### What is Expected?
可以使用
### What is actually happening?
不能使用
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Assessment
This issue has not been assessed yet.