🚨 Error: null is not an object (evaluating 'a.__AMP_TOP')

Details
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**Error report:** [link](http://go/ampe/CNy-7d70k6SImAE)
**First seen:** Oct 15, 2020
**Frequency:** ~ 55/day

Stacktrace
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Error: null is not an object (evaluating 'a.__AMP_TOP')

    at win (src/service.js:285:9)

    at win (src/service.js:159:21)

    at defaultView (src/service.js:314:31)

    at elementOrAmpDoc (src/service.js:224:27)

    at builtins/amp-pixel.js:91:46

Notes
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`@cramforce` modified `src/service.js:285-285` in #23767 (Aug 7, 2019)
`@choumx` modified `src/service.js:153-162` in #10131 (Jun 28, 2017)
`@rsimha` modified `src/service.js:313-316` in #21212 (May 16, 2019)
`@dvoytenko` modified `src/service.js:223-224` in #30634 (Oct 15, 2020)
`@rsimha` modified `builtins/amp-pixel.js:84-91` in #21212 (May 16, 2019)

**Seen in:**
- 10-16 Nightly (2313)

**Possible assignees:** `@dvoytenko`, `@cramforce`

/cc @ampproject/release-on-duty

Start with the linked stack-trace locations in src/service.js, especially lines 285, 159, 314, and 224, then inspect builtins/amp-pixel.js:91. Use the linked error report and the nightly occurrence details to understand when the null value appears. Done means identifying the triggering path and addressing the reported null access with evidence that the error no longer occurs.

Written by the indexing model from the issue text.