amp-ad - Uncaught TypeError: Cannot read property 'create' of undefined

[fox94610]

Directly related to this "closed" issue:
#17997

I know this is a 'silent' error that I guess doesn't have any user facing side effects, but I thought y'all should know it's still out there. Go to this page and you'll see it:
https://www.babycenter.com/amp/0_breastfeeding-exclusively-for-3-months-decreases-risk-of-chr_40005552.bc

If you don't see it the first time, reload browser, it's dependent on what ad is being served.

[torch2424]

cc @ampproject/wg-ads

This would make a great fixit item!

Thank you for submitting the issue @fox94610 😄

[zhouyx]

cc @jridgewell

[jridgewell]

Dupe of #17997.