Initialization of custom elements in friendly iframe causes error to be logged.

[cramforce]

This repros on any page with an A4A ad that contains at least one custom element.

v0.js:42 Uncaught TypeError: Cannot read property 'create' of undefined
    at new uj (v0.js:42)
    at new Cf (document-register-element.node.js:294)
    at document-register-element.node.js:284
    at Wb (document-register-element.node.js:291)
    at Sl (amp-img.js:152)
    at Kl.f.installExtensionsInChildWindow (vsync-impl.js:438)
    at friendly-iframe-embed.js:209

Example URL https://amp-20minutos-es.cdn.ampproject.org/v/s/amp.20minutos.es/noticia/3357093/0/nueva-portada-revista-time-creada-98-drones/?amp_js_v=0.1&usqp=mq331AQECAEoAQ%3D%3D

This appears to be harmless but spams the console. I don't completely follow why it is logged OR why it is harmless :)

[oliverfernandez]

This is happening in AMP Shadows as well, whenever an AMP-AD with Doubleclick Fast Fetch implementation is used

https://www.thelondoneconomic.com/

We have detected this since 2 weeks ago approximately

[gabelew]

We are getting the same error as we also have amp-img in our ad. Is there any update to this issue?

[jridgewell]

This will be fixed by #17205.