If you're just looking for a solution to the issue, here it is: quit Safari, copy the following command to the clipboard, open the Terminal app (located in the Utilities subfolder of the Applications folder), and paste the command.
/System/Library/Frameworks/CoreServices.framework/Frameworks/LaunchServices.framework/Support/lsregister -f -R /Applications/Safari.app
That's it, problem solved! The rest of this blog post will attempt a diagnosis of the problem.
Call the lsregister command above with no arguments to see usage information for the tool. I created an alias in my .bash_profile for convenience.
alias 'lsregister=/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister'
https://lapcatsoftware.com/articles/disappearing-safari.html