Do you think this is a problem with MacOS Mojave? Please help me.
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.