🚨 Error: undefined is not an object (evaluating 'c.ampdoc')

Details
---
**Error report:** [link](http://go/ampe/CLad-eS8vsD18wE)
**First seen:** Apr 9, 2020
**Frequency:** ~ 1,349/day

Stacktrace
---

Error: undefined is not an object (evaluating 'c.ampdoc')

    at ampdoc (src/multidoc-manager.js:516:11)

    at closeShadowRoot_ (src/multidoc-manager.js:72:54)

    at extensions/amp-next-page/1.0/page.js:193:11

    at page (extensions/amp-next-page/1.0/service.js:418:19)

    at extensions/amp-next-page/1.0/service.js:411:13

    at hidePreviousPages_ (extensions/amp-next-page/1.0/service.js:344:8)

    at extensions/amp-next-page/1.0/service.js:335:24

    at extensions/amp-next-page/1.0/service.js:542:8

    at relativePos (extensions/amp-next-page/1.0/visibility-observer.js:310:29)

    at bottomSentinelPositionChanged_ (extensions/amp-next-page/1.0/visibility-observer.js:98:8)

    at position (src/service/position-observer/position-observer-worker.js:109:20)

Notes
---
`@wassgha` modified `src/multidoc-manager.js:491-532` in #25629 (Dec 10, 2019)
`@wassgha` modified `src/multidoc-manager.js:32-114` in #25629 (Dec 10, 2019)
`@wassgha` modified `extensions/amp-next-page/1.0/page.js:184-193` in #26296 (Jan 23, 2020)
`@wassgha` modified `extensions/amp-next-page/1.0/service.js:413-429` in #26296 (Jan 23, 2020)
`@wassgha` modified `extensions/amp-next-page/1.0/service.js:410-411` in #26296 (Jan 23, 2020)
`@wassgha` modified `extensions/amp-next-page/1.0/service.js:344-345` in #26296 (Jan 23, 2020)
`@wassgha` modified `extensions/amp-next-page/1.0/service.js:328-335` in #25636 (Dec 10, 2019)
`@wassgha` modified `extensions/amp-next-page/1.0/service.js:541-545` in #26296 (Jan 23, 2020)
`@wassgha` modified `extensions/amp-next-page/1.0/visibility-observer.js:294-381` in #26778 (Apr 9, 2020)
`@wassgha` modified `extensions/amp-next-page/1.0/visibility-observer.js:98-98` in #26778 (Apr 9, 2020)
`@zhouyx` modified `src/service/position-observer/position-observer-worker.js:106-136` in #10813 (Sep 8, 2017)

**Seen in:**
- 08-22 Control (0050+1)
- 08-22 Nightly-Control (0050+1)
- 08-22 Stable (0050+1)

**Possible assignees:** `@wassgha`, `@zhouyx`

/cc @ampproject/release-on-duty