Finder Problems? (Error Message: 'The application “Finder.app” is not open anymore.')

Today, with no apparent cause, I started getting the following message when I tried to open a folder through a spotlight search: 'The application “Finder.app” is not open anymore.'

This is obviously not true, as you can see from the following screenshots. which shows me doing a search in Spotlight for the word "Bluetooth", followed by the popup error message I get as a result. In both, a Finder window showing the results of searching for results with "bluetooth" in their name, is in the background (when this started I was looking for the user manual for my bluetooth headset).

Note however that if I try to open one of the files in the Bluetooth folder directly, I have no problem.

Finder and Spotlight - specifically the part of Spotlight that finds local files and folders (presumably going through Finder) - have also been acting weird. Sometimes searches are slow. Sometimes Spotlight results will be missing all local files or folders, although they will appear later if I let the Spotlight "window" remain active long enough. Etc. A lot of this other stuff is hard to describe exactly because it comes and goes and is hard to produce reliably.

I have already tried relaunching Finder and that did not work.

Thanks!