Can't close Get Info box in Calendar
macOS Tahow 26.0.1. I had this same exact issue as this person 9 months ago:
Somewhere along the line an Info box was opened in my Calendar app. It's a blank window titled "Info" and the red close button doesn't make it go away. Restarting Calendar doesn't work. Restarting the Mac doesn't work. Force quitting Calendar doesn't work.
I fixed it by deleting preferences (moving to the desktop):
mv ~/Library/Preferences/com.apple.iCal.plist ~/Desktop/
MacBook Air 13″