I did a search and found that this seems to be a somewhat common problem, or at least a problem that is unknown. A few solutions were proposed, one of which seemed to work for most people -- reboot your computer three times -- but I was working and didn't want to take the time at the moment to try that. I kept searching. And I found a solution from a tech website that I tried...and it worked.
Since I figure that other people use Avast! anti-virus, they too might come across this same problem -- if not now, then perhaps in the future. Here's the solution. (Will it work each time? Who knows, but it worked here.)
It's odd, I don't know why it works, but...it worked.
- Right-click the avast! icon in the System Tray.
- Hover your mouse over "avast! shields control."
- Click Disable for 10 minutes. (You'll get a warning that you're unprotected.)
- Click on the Windows Taskbar to exit from the avast! icon menu, if necessary.
- Repeat Steps 1 & 2.
- Click Enable all shields.
- Check Windows Action Center: the security notification should be gone now.