🚨 Error: Cannot read property 'ampdoc' of undefined

Details
---
**Error report:** [link](http://go/ampe/CL27g42-x_HsfQ)
**First seen:** Apr 11, 2020
**Frequency:** ~ 1,805/day

Stacktrace
---

Error: Cannot read property 'ampdoc' of undefined

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

    at closeShadowRoot_ (src/multidoc-manager.js:160:13)

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

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

    at previousPages (extensions/amp-next-page/1.0/service.js:406:6)

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

    at forEach (extensions/amp-next-page/1.0/service.js:335:4)

    at updateVisibility (extensions/amp-next-page/1.0/service.js:221:8)

Notes
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`@wassgha` modified `src/multidoc-manager.js:491-532` in #25629 (Dec 10, 2019)
`@wassgha` modified `src/multidoc-manager.js:138-172` 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:405-406` 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:217-223` in #26778 (Apr 9, 2020)

**Seen in:**
- 08-15 Nightly-Control (0009+1)
- 08-22 Beta (0050+1)
- 08-22 Experiment-C (0050)
- 08-22 Experimental (0050)
- 08-29 Nightly (0323)
- +14 more

**Possible assignees:** `@wassgha`

/cc @ampproject/release-on-duty