Apple - 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:

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

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

^{NOTE: I am in no way affiliated with EasyFind.}