Only update state if we have a valid reference

[javidalkaruzi]

Description
This should resolve #24495.

If a block is a child of a group block, it will have a parent selector that is shown when we hover over the block switcher and hidden when we move from it. The function to hide this is called in a setTimeout callback. If the block is unmounted, we still attempt to hide the parent selector via the setTimeout callback. By the time the callback gets executed, it is possible that the component would have already been unmounted.

How has this been tested?

• Ensure that the existing unit tests were passing
• Manually test and observed in the browser

Types of changes

Bug fix

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR.

[talldan]

I had some trouble reproducing the original issue, but having said that I can see how it could logically happen, and this fix doesn't cause any issues, so it seems good to have the added guard.

Thanks for working on this!