If a simple reboot doesn't fix the problem, reboot your Mac in safe mode and make the change to Safari as the default browser in that mode. Then restart normally and see if that has any positive effect.
How to use safe mode on your Mac - Apple Support
If that doesn't fix the issue, then restart your Mac in recovery and run Disk Utility First Aid on the startup drive.
How to repair a Mac storage device with Disk Utility - Apple Support
Note- you cannot repair the startup disk unless you are booted in recovery mode.
Boot in recovery and then launch Disk Utility. Select View > Show All Devices and run First Aid on the drive volumes, container and the media device itself.
Then reboot and reevaluate the problem.