Check if editor is still mounted before trying to focus

[delijah]

Description
The DOMEditor.focus method has a retry mechanism built in. It can happen, that while retrying to focus the dom node of the editor, meanwhile the editor has been unmounted, which will result in an error when calling DOMEditor.toDOMNode(editor, editor).

This PR adds a check on top of the DOMEditor.focus method, to check if the editor is still mounted, if not return and do not retry to focus the editor dom node anymore.

Issue
Fixes: #5915

Context
This error happened in our unit tests. I guess it's hard to reconstruct in the browser, since it is happening in the milliseconds area.

Checks

• The new code matches the existing patterns and styles.
• The tests pass with yarn test.
• The linter passes with yarn lint. (Fix errors with yarn fix.)
• The relevant examples still work. (Run examples with yarn start.)
• You've added a changeset if changing functionality. (Add one with yarn changeset add.)

[changeset-bot]

🦋 Changeset detected

Latest commit: d0c535a

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
slate-dom	Patch

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