SystemExtr quit unexpectedly This message keeps on popping up again and again. I tried restarting my Mac and did no good. I thought it was steam causing these problems but deleting it did not make any difference. 

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.