Comment 34 for bug 1891680

> "Could not install 'grub-pc'. The upgrade will continue but the 'grub-
> pc' package may not be in a working state. Please consider a bug report
> about it. Installed grub-pc package post-installation script subprocess
> returned error exit status 1."

This is not evidence that this bug is not fixed. If your grub was misconfigured prior to upgrade, this is the INTENDED outcome: the package upgrade breaks, but importantly, it doesn't break in a way that leaves your system unbootable.

You should be able to run 'sudo dpkg-reconfigure grub-pc' to reconfigure grub to properly point at your boot disk, so that it is actually possible to upgrade grub on your system, and configure from there.