If this error appears and you cannot click back into your Dashboard, go back to yourblogname.com/wp-admin, which will reload your dashboard. It will redirect you to your login page if you are not logged in.
This error typically happens when you access the Customizer directly from the Required Plugins Installer, after you have installed and activated all recommended plugins. The Customizer remembers the last page you accessed and tries to return you there when you exit, but the Required Plugins Installer page presents this error because there is nothing left for you to install or activate.
Bottom line: it’s no biggie and a rare sequencing bug that’s unfortunately unavoidable. Head back to your dashboard, and you should be good to go!