error: symbol 'grub_file_filters' not found [upgrade Ubuntu18.04 to 20.04]

What DID NOT WORK

1. I had the same issue and above solution by @rapid3642 DID NOT WORK. The CHROOT case was not the right case for me because I could use F12 and able to log in to linux without issue. I also tried running the above commands as chroot on ubuntu system / partition directly ( without the mounting part as I had ubuntu installed and working )
2. Rescatux also did not work although it seems to be the utility which was supposed to be a clean gui.

I used a comment from Kristina Kovacek to get the idea of what we need to do.

What did work for me is the boot-repair-disk utility. Funnily enough it did not work in the default mode ( the Recommended Repair section ) but I had to change following

Advanced Options -> Grub location

1. Change OS to boot by default to the Linux ( yes you can log into windows but you need to keep linux as Master )
2. Untick Spearate /boot /boot/efi partition
3. Place grub into <your linux partition such /dev/sda>

I also removed one other option making sure that by default the GRUB looks into windows I dont remember what it was ( even though I executed the procedure just few moments ago

NOTE that there are instructions that you need to follow and commands that you need to execute manually which also involves purging the grub-install from the linux partition ( which in retrospect might have sufficed. )