GH-2071 A/B testing console error

[benstrumeyer]

• Have you followed the guidelines in CONTRIBUTING.md?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you added an explanation of what your changes do?
• Does your submission pass tests?
• Did you lint your code prior to submission?

• pm is undefined when logging into plus or premium account and the user has a midnight promo modal A/B test

Steps to replicate:

• Set conf.hub_promo_variant = 'midnight' in background#setupHubPromoABTest()
• Load the extension (automatically routed to the Home View with Premium promo modal shown)
• Log into a plus account and be redirected to the HomeUpgradePlanView
• Click the Home View and notice the error in the console

Ticket: https://ghostery.atlassian.net/browse/GH-2071