update nested section throw error `index out of range.`

[zhixzhan]

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Describe the bug

 > !# @enableSections = true

# CheckTodo
## CheckUnreadTodo
- check my unread todo
- show my unread todos

@ simple todoTitle

## CheckDeletedTodo
- check my deleted todo
- show my deleted todos

@ simple todoSubject

update CheckTodo with content:

# CheckTodo
## CheckUnreadTodo
  - please check my unread todo
  - show my unread todos

@ simple todoTitle

see error.

To Reproduce

Steps to reproduce the behavior:

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

Expected behavior

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Additional context

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[bug]

[feich-ms]

closed since it was caused by another PR #1002 and the issue is resolved there.