function to reset the selection if it's invalid

[swapanmandi]

fix #6805

This PR fixes an issue where the Slate.js editor in Volto attempts to set the selection to a non-existent path.

Changes Made:
Added Selection Validation

Before setting the selection, the editor checks if the target path exists.
If the path is invalid, it resets the selection to the start of the block.
Ensured Synchronization with defaultSelection

Clears defaultSelection when the block updates to prevent stale references.
Added Debugging Logs for Document Structure

Logs editor.children before applying selection to help debug future issues.

[boring-cyborg]

Caution

The Volto Team has suspended its review of new pull requests from first-time contributors until the release of Plone 7, which is preliminarily scheduled for the second quarter of 2026.
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If you haven't done so already, read
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Contributing to Plone,
First-time contributors, and
Contributing to Volto,
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Welcome to the Plone community! 🎉

[mister-roboto]

@swapanmandi you need to sign the Plone Contributor Agreement to merge this pull request.

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[sneridagh]

@swapanmandi if you don't sign the CLA, we will have to close this PR.

[swapanmandi]

I signed

[stevepiercy]

@swapanmandi did you receive and accept an invitation, as described in https://6.docs.plone.org/contributing/index.html#sign-and-return-the-plone-contributor-agreement ?

If you didn't receive the invitation, then follow the instructions on that page.