Fix access to undeclared variable when trying to create invalid Auth provider

Summary: Ref T6971. This fixes the error the user reported. Not sure what's up with the root cause of their issue.

Test Plan: Went to /auth/config/new/asdfqwer/ and got a 404 instead of an exception.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T6971

Differential Revision: https://secure.phabricator.com/D11388