fix: Cannot read property level of undefined

[sy-records]

Summary

fix #1356

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Repo settings
• Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

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If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated
• Related tests have been updated

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[anikethsaha]

Optional Suggestion : Can we have some tests

[Koooooo-7]

could you give more details about this change?

[sy-records]

@Koooooo-7 You can try it yourself, it causes the sidebar to fail to render here.

Run the following README.md:

# Heading 1

Test

## Heading 2

### Heading 3A

Lorem ipsum

#### Heading 4A

##### Heading 5A

Lorem ipsum

### Heading 3B

Lorem ipsum

#### Heading 4B <!-- {docsify-ignore} -->

##### Heading 5B

Lorem ipsum