The Default Post Logout URL is not set

I got this after logging out from the dashboard on https://dashboard.staging.stackspin.net (which redirected me to https://sso.staging.stackspin.net/oauth2/fallbacks/logout/callback):

The Default Post Logout URL is not set which is why you are seeing this fallback page. Your log out request however succeeded. You are seeing this page because configuration key urls.post_logout_redirect is not set. If you are an administrator, please read the guide to understand what you need to do. If you are a user, please contact the administrator.