That’s right, Grub Customizer can break things really badly–it can even mess up your bootable snapshots.
It’s kind of tricky to remove; uninstalling the package is not enough. This is from another distro, but it has some good guidance for removing Grub Customizer if you would like to take a look:
You can accomplish this safely with Grub’s GRUB_TOP_LEVEL option. See this post here: