[Bug]: import from v3 failed

[sparrowyang]

Describe the bug

To reproduce

1. Go to v4
2. Click on import from massscode v3
3. See error

Version

4.0.0-beta.1

OS

win 10

Validations

• Follow our Code of Conduct
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion.

[antonreshetov]

For some reason you have a missing index property in folders for some item in db.json. Try opening the file and see exactly where that index is missing and add an arbitrary one.