TYPO3 Core

When adding a new login form to a fresh site, several exceptions are thrown as the redirect URL is null and not a string. The exceptions come from RedirectUrlValidator::isValid().

Steps to reproduce:

1. create a new site with a basic template
2. create a folder for users, configure the constant styles.content.loginform.pid with that folder's uid
3. insert a login form on the start page, w/o any configuration in the flexform
4. open the start page – boom, you're greeted with an exception

Bonus track: if the constant is not configured, the page is rendered, but the login form shows "oops, an error occurred" because the PID is not configured (as you then can see from the log).