Posting this here in case it helps others and if someone more familiar with the project feels it should be a PR, maybe convert it into one.
My Pi-Hole instance somehow ended up with a client in the database without a name. I don't know how that happened, but it did.
This resulted in an error message popup in the web UI that read:
DataTables warning: table id=clientsTable - Invalid JSON response. For more information about this error, please see 1. Warning: Invalid JSON response
It also resulted in a repeated entry in the log:
Trying to access array offset on value of type bool in /var/www/pihole/admin/scripts/pi-hole/php/groups.php on line 208
I fixed the problem by editing admin/scripts/pi-hole/php/groups.php and changing line 208 from:
$res['name'] = $name_result['name'];
$res['name'] = is_array($name_result) ? $name_result['name'] : '';
Once I did this, I could clearly see the client without a name and work within the system again.
I hope this helps the next guy/gal and if someone more confident this wasn't a stupid move wants to convert this into a PR, that would be wonderful.