Describe the bug
Just updated to 4.5.1. Can't create any block on a new page or new post - it will just show this "Start writing text or HTML" box (please see the screenshot attached); the "add block" button at the top left of the screen is disabled (please see the screenshot attached). When editing posts or pages, the same thing happens, all content is showed as HTML. Browser console shows no errors or warnings so I'm completely lost.

To Reproduce
Steps to reproduce the behavior:

1. Update Gutenberg to 4.5.1.
2. Add a new page or post.
3. Try to add a block (text, image, list, whatever).

Expected behavior
The usual Gutenberg behavior - Ability to add a block and edit them visually, like 4.4(?) version.

Screenshots

Desktop (please complete the following information):

• OS: Fedora GNU/Linux
• Browser Firefox
• Version 63.0.1 (64 bits)