Gutenberg: Custom HTML "An unknown error occurred" mixed content error

[Forecaster]

Describe the bug
When trying to use the custom HTML block the following is shown:

It appears to be unable to load the code editor because it's trying to load it via http on my https site. So the script is blocked.

To Reproduce
Steps to reproduce the behavior:

1. Have SSL enabled site
2. Try to use Custom HTML block
3. Observe Mixed Content error hopefully?

Additional context
Using Gutenberg 3.3.0
Just updated in fact and unfortunately the issue remains.

Running WordPress 4.9.7 on an ancient Ubuntu Server 12.04.5 box, with a very old PHP version (unfortunately)