Demo doesn't load after activation following 4.9.8 upgrade

[ChrisHardie]

Describe the bug
After upgrading to 4.9.8 and installing/activating Gutenberg, I'm redirected to /wp-admin/post-new.php?gutenberg-demo. This shows the wp-admin sidebar and top admin bar, but the main content area is blank. No Javascript errors in the console.

To Reproduce
Steps to reproduce the behavior:

1. Upgrade WordPress to 4.9.8
2. Install Gutenberg
3. Activate Gutenberg
4. Be redirected to /wp-admin/post-new.php?gutenberg-demo and see a blank content area.

Expected behavior
I expected to see the demo.

Desktop (please complete the following information):

• OS: macOS
• Browser Safari
• Version 11.1.2 (13605.3.8)

[designsimply]

Thank you for the report! Consolidating at #8455 since there are more replies there already and noting a comment with a possible solution for reference:

So this could happen for several reasons:

• Do you see any errors in the JavaScript console of your browser?
• Do you see any failing XHR requests in the console?

It can also happen without any of those and in that case, we may have found the issue here https://meta.trac.wordpress.org/changeset/7593 (sometimes refreshing the page fix it). Hopefully, we'll land a fix soon.

^{Source: #8455 (comment)}