SystemExtr quit unexpectedly
No, this is not a problem with macOS Mojave. I've seen this before and it was caused by an app hiding within the Library folder. 

Unfortunately I don't recall the app, but I do recall how we got to the bottom of it. In summary:


Install EasyFind (it's free from the Mac App Store)
Search for SystemExtr
Identify the file (app) responsible
Remove it


As mentioned, from memory it was sitting in the Library folder.

NOTE: I am in no way affiliated with EasyFind.