Stuck Apple Mail - how to reset?
You could perhaps try deleting the saved state files, then on next launch it would return to a default view.
The advantage is that no other settings would be changed - nothing lost.

~/Library/Saved Application State/com.apple.mail.savedState

Because it's an alias, don't delete the folder itself, just the contents.

That would potentially allow you to empty the Trash folder without having to first open it.