Steps to reproduce

1. Disconnect from the internet (e.g. toggle wifi)
2. Start Jenkins (e.g. docker run -p 8080:8080 jenkins/jenkins)
3. Log in, skip everything in setup wizard ("Skip Plugin Installations", "Continue as admin", "not now", "Start using Jenkins")
4. Go to Manage Jenkins » Plugins » Download progress and click the first link labeled "configure HTTP proxy?"

Expected result

The link works

Actual result

The link takes you to http://localhost:8080/manage/pluginManager/updates/nullmanage/pluginManager/advanced because the Root URL is null at this point (and the fallback to request URL didn't work).

Notes

Introduced in https://github.com/jenkinsci/jenkins/pull/7992 

The first error on the Download progress page was logged when Jenkins started, so it would not pick up a URL configured during the wizard.