Desktop: Fix crash when closing a secondary window with the Rich Text Editor open

[personalizedrefrigerator]

Summary

This pull request fixes a "Joplin encountered a fatal error and could not continue" screen caused by calling .remove() on a TinyMCE editor after its parent window has been closed. With this change, .remove is not called on an editor with no parent window.

Note

The issue does not seem to be present in Joplin v3.2. The issue doesn't seem related to the Electron or TinyMCE upgrades — it's still present after downgrading to TinyMCE v5 and Electron v32. Edit: Based on a git bisect, the does seem related to the commit that upgraded from TinyMCE v5 to v6. I suspect that the issue is related to one of the changes made to the editor setup to prepare for TinyMCE v6.

Error message

Joplin encountered a fatal error and could not continue.

To report the error, please copy the *entire content* of this page and post it on Joplin forum or GitHub.

If the error persists you may try to [restart in safe mode](https://github.com/laurent22/joplin/compare/dev...personalizedrefrigerator:pr/desktop/fix-crash-on-close-secondary-window?expand=1#), which will temporarily disable all plugins.

Message
Failed to execute 'acceptNode' on 'NodeFilter': The provided callback is no longer runnable.

Testing plan

1. Switch to the Rich Text Editor.
2. Open a note in a new window.
3. Close the new window.
4. Verify that Joplin's main window doesn't show an error screen.