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Photo.app does not open when clicked on in the dock or in the list of apps. Message comes up "The application "Photos.app" is not open anymore". How can I re-open it?
I'd try clearing the saved state files.
~/Library/Containers/com.apple.Photos/Data/Library/Saved Application State
Find the folder com.apple.Photos.savedState and delete it.
That should give Photos a fresh start.