Fix hasDOMNode throwing error when editor is not mounted

[gbalint]

Wrap the toDOMNode call in a try-catch to gracefully handle the case where the editor is not yet mounted. In this case, the target cannot be part of the editor, so returning false is the correct behavior.

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[12joan]

My concern is this might risk masking genuine issues. Have you considered introducing a new DOMEditor.isMounted(editor) query that you can use prior to calling hasDOMNode instead? The logic for this could be refactored out of DOMEditor.focus, which is currently using !EDITOR_TO_ELEMENT.get(editor) to determine if we're mounted or not.