amp-analytics doesn't find an element

## What's the issue?

I am using `amp-analytics` to track my ads, but when using `amp-analytic` with `amp-auto-ads` it doesn’t track for the elements created after the `amp-analytics` has been loaded.

I believe this is happening with all amp extensions that dynamically loads elements on the page and not only `amp-auto-ads`.

Error message

> Element "XXX" not found: null

## How do we reproduce the issue?

I created 2 environments trying to simulate a delay on the extension so I can reproduce the error that occurs depending on user’s network:

1 – I delayed in 1 second the `amp-analytics` request: https://delay-amp-analytics.denakop.com/amp.html

2 – I delayed in 1 second the `amp-auto-ad` request: https://delay-amp-auto-ads.denakop.com/amp.html

When delaying the `amp-analytics` the error will not occur because the selector exists.

When delaying `amp-auto-ad` the `amp-analytics` will throw an error because when it is loaded the `` elements is not created yet

## What browsers are affected?

I managed to reproduce the error on all major browsers and devices.

## Which AMP version is affected?

I believe this issue is happening in all AMP versions