[BUG] WriteToFile fails to create backup file before overwriting existing content

**BUG REPORT**
1. Please describe the issue you observed:

- What did you do (The steps to reproduce)?
尝试向一个已存在的非空文件写入内容。

- What did you expect to see?
内容成功写入指定位置，原文件内容写入到备份文件中。

- What did you see instead?
备份文件内容为空

2. Please tell us about your environment:

3. Other information (e.g. detailed explanation, logs, related issues, suggestions on how to fix, etc):
应该是`err==nil` 时写入