I added a Second Volume to my Main APFS Container to install and try Mojave (I'm running High Sierra). I decided that Mojave is not for me and I erased the volume from Disk Utility. My Mac still tries to boot into Mojave by default when I restart my computer (it fails because that volume doesn't exist anymore).
In order to go into my High Sierra OS, I always have to boot with Alt pressed to choose the boot disk. If I don't do this, I get the "🚫" icon.
The weird thing is that this volume was indeed erased and does not appear on Disk Utility nor on the Startup Disk menu in System Preferences.
How can I fix this? I want to remove the "Mojave" Volume from the boot disks so that my computer will start normally when I turn it on.
EDIT: I seem to have found something useful in this post, but it is for people who still haven't erased the volume from Disk Utility. I don't know how to do it in my case, where I already erased it.