"Show Side Preview" not working. How do I get it back?

Catalina v10.15.5 Mail v13.4

I unchecked "Show Side Preview" in the view menu and the preview was then at the bottom. My intent was to be able to select messages without reading them so I dragged that divider to the bottom.

Now I want to restore the :Side Preview" and it will not come back. Even if I pull the divider up before checking the menu item, it will not come back.