Can't close pop up window

Hello. When trying to run Libre Office, which has worked great till now on Mac, I get a popup that says “The last time you opened LibreOffice, it unexpectedly quit while reopening windows. Do you want to try to reopen its windows again?” and I can’t close it.

Please have a look at → LibreOffice startup always asking reopen windows

See also: Tagged “restore-windows”