Once you have that backup (ideally, more than just one backup) the following are a few suggestions to consider, in order from simple and fast to more time-consuming and tedious.
Quit the Safari app if it open. Press and hold a Shift key while launching Safari from Applications or the Dock.
- That action opens Safari in windowed mode (not full screen) with no previously opened windows or tabs.
Release the Shift key when a Safari window opens, or it crashes again.
If that doesn't immediately fix it, create a new, temporary User Account according to Add a user or group on Mac - Apple Support. Log out of your usual Account and log into that temporary one. Skip the steps regarding various preferences: select Skip, Set Up Later, Not Now, etc. Determine if you can launch Safari in that User Account. If the same problem occurs in that User Account, reinstalling macOS is justified: How to reinstall macOS - Apple Support.
If the problem does not occur in that temporary User Account, then fixing the problem becomes more challenging. Write back for instructions. Leave that temporary User Account alone for now, because one fairly quick fix would be to copy its ~/Library/Safari folder and replace your usual ~/Library/Safari with its contents. When doing that, the importance of a Time Machine backup becomes critical.