Don't display "SHOW COMPLETED TODOS" when there aren't any.
Make the display conditional. When "SHOW COMPLETED TODOS" appears, the subtext is that there are some todos to be shown. But there may not be any.
Also, make it easier to dispose of unwanted or completed todos. The extra steps necessary to fully eliminate a todo just wasted our time.
As an alternative, consider providing an undo feature. Oops, I deleted something! Undo it...