OK, I tested for you.
Running PHP 7.2.9. (PHP-FPM)
This seems to do the trick on my system… but I didn’t experience the 500 error others complained about. Instead, when I ran the upgrade, everything looked fine, front & back, but the “Kirki” not found fatal error was registered in my logs.
I added the line of code at #1142 and the error was generated one more time a few seconds later, but no more after that. I’ve been starting, navigating, and exiting the customizer, so maybe the first error was just because of a cached page that needed to be refreshed. In any case, no more errors being logged, and no evident problems anywhere else on my site. (But then, unlike others… my site didn’t break in the first place… I wouldn’t have known there was a problem but for the fact that I was testing for you and closely watching the error log each step of the way).
So I think you are probably on the right track.