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.