I have exactly the same problem.
I tried to uninstall and reinstall the software. It didn't solve anything.
It's doing this problem since I installed the last update of Premiere Pro.
It's a known problem. They say they are working on it :
The only solution I found is going back to the previous version. If you haven't deleted anything yet on your Mac, it may still be there (click on the down arrow, beside the app's name in the CC window and find "Other versions".
I hope this helps.
Premiere Pro is not listed as a program having issues with High Sierra.
As It has been said in the link I posted above, I think it's more likely a problem within the latest version of Premiere than a High Sierra problem.
Thanks for the answers !
I downloaded a former Premiere Pro version, and it's working again.
in any case I think we need to wait for the next bug fix update for solving this .
Update to 12.1.1 and you should be clear of the issue. Let us know.