I have the same problem.
I have WordPress 5.0.1. My WordPress site is at http://www.waterwayroutes.co.uk/blog
Everything was working until I upgraded Catch Box to 4.8. Since then I cannot edit any posts and I just get a white screen in Gutenberg. I can edit with the classic editor in text mode (if I enable that plugin) but the visual mode for that also gives a white screen.
I have tried disabling all plugins and that doesn’t help.
I have tried deleting the Catch Box theme and reinstalling and that doesn’t help.
I tried changing themes (to Twenty Nineteen) and everything started working normally, even with all plugins re-enabled. When I changed back to the Catch Box theme the editors stopped working again.
I tried on two different computers (both Windows 10) and using Chrome, IE and Firefox and all have the same problem. The same problem occurs using my Android phone and Android tablet too.
All this strongly suggests the problem is with Catch Box 4.8.
Please fix it quickly, or at least post simple instructions about how to go back to the previous Catch Box temporarily.