Start by using Activity Monitor >> View >> View ALL Process. Check in Memory tab for some process that seems associated with this pop up. Then lighting the process and Force Quit using the Top Bar of Activity Monitor . The tool with the X in it.
Baring that effort >>
1 - Suggest restarting in Safe Mode. This will perform a Disk Repair, clear cache files and only load Apple Software, extensions and fonts. The boot up will be slow and can take some time - Normal.
2 - Does the issue present in this mode ?
3 - If not - there could be something in the main User Account playing up. To further isolate this - Set up users, guests, and groups on Mac. Then log out of the Main User account and log into the dummy account and test again if the issue persists.
4 - If the issue is present in the dummy account - then this appears to be a System Wide issue.
If the issue is still presented please report back for additional assistance