##What's the issue?
Error with Amp-video-iframe

Briefly describe the bug.
Cannot read property 'contentWindow' of null.

How do we reproduce the issue?

1. Opt into Canary/RC
2. Open javascript console
3. Go to page https://www.google.com/amp/s/www.cnet.com/google-amp/news/iphone-11-pro-11r-max-new-specs-features-price-expect-apple-every-rumor-2019-pro/
4. Hit back button on browser

What browsers are affected?

macOSX Chrome Version: 76.0.3809.100

Which AMP version is affected?

Canary/RC: 1908150025400

Video: https://drive.google.com/open?id=1jCBkHV9Qk7Ysirb3a5y95YmPSAjV1zCK