Fix a bad call when prefilling ApplicationSearch from ?projects=some_slug

Summary: Fixes T10299.

Test Plan:

• Visited /maniphest/?projects=x locally, where x is some valid project slug.
• Before patch: Fatal on requireViewer() call.
• After patch: Works correctly, filling the correct project into parameters.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10299

Differential Revision: https://secure.phabricator.com/D15214