"SafariBookmarksSyncAgent quit unexpectedly" Keeps Popping Up and Cannot Open Safari
What ended up working for me was restarting the computer in safe mode. 

Here's an explanation from an apple discussions post. 


  Certain caches maintained by the system will be rebuilt.
  
  Safe mode is much slower to start up than normal. The next normal
  startup may also be somewhat slow.
  
  When the login screen appears, restart as usual (not in safe mode) and
  test. There's no need to log in while in safe mode.
  
  Note: If FileVault is enabled in OS X 10.9 or earlier, or if a
  firmware password is set, or if the startup volume is a software RAID,
  you can’t start in safe mode. In that case, ask for instructions.


From: https://discussions.apple.com/thread/7069199