Remove the multiplexer on window reload/close

[FinnChen]

What it does

Fixes #11600.

How to test

• Run the electron example
• Press cmd+r to reload the window
• Click the New Window in the menu, and it does not work.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[vince-fugnitto]

I can confirm that the changes work well for me, and fix an issue where I was unable to perform open folder (which creates a new window) on electron 👍

[msujew]

Looks good to me as well 👍