Until there’s an update to Invoice Ninja with the latest version of Laravel, here’s an easy way to revert back to PHP 7.2 or previous (assuming you only updated, but didn’t uninstalled the previous versions).
Run sudo update-alternatives --config php to list the versions of PHP available on your system. Select the option for 7.2 (or whichever previous version you have installed).
Do the same for your PHP extensions with sudo update-alternatives --config phar.
That should take care of the issue until there’s an update. The usual “your mileage may vary”, “this might be slightly different on your system”, and “I take no responsibility for what happens” disclaimers apply here.
AFAIK, Laravel 5.8 takes the changes into account, so it should just be a matter of including the newer version in the next update. But I’m not sure how that all plays together, and when I just copied over the newer /framework/src/Illuminate/ my whole install broke until I reverted to the backup I’d made. I’m happy to stick with PHP 7.2 until it’s all sorted.