Failed to execute 'postMessage' on 'DOMWindow': https://www.youtube.com !== http://localhost:9000

This is the error message that I get:

Failed to execute 'postMessage' on 'DOMWindow': The target origin provided('https://www.youtube.com') does not match the recipient window's origin ('http://localhost:9000').

I've seen other similar problems where the target origin is http://www.youtube.com and the recipient origin is https://www.youtube.com, but none like mine where the target is https://www.youtube.com and the origin is http://localhost:9000.

1. I don't get the problem. What is the problem?
2. How can I fix it?