How to resolve messed up GPT (FFFFFFFF-FFFF-FFFF-FFFF-FFFFFFFFFFFF) from Ubuntu without data loss
Enter the following command. This will change the MacOS Catalina partition back to an APFS container.

sudo sgdisk -t 2:7C3457EF-0000-11AA-AA11-00306543ECAC /dev/sda


Next, you will need to reboot. Immediately after rebooting, hold down the option key until the Startup Manager appears. Hold down the control key before selecting macOS from the Startup Manager choices.