🚨 Error: Cannot destructure property 'sourceElement' of 'Z(...)' as it is undefined.

Details
---
**Error report:** [link](http://go/ampe/CJPGkJ_U9ZC_RQ)
**First seen:** May 12, 2021
**Frequency:** ~ 2,577/day

Stacktrace
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Error: Cannot destructure property 'sourceElement' of 'Z(...)' as it is undefined.

    at sourceElement (extensions/amp-lightbox-gallery/0.1/amp-lightbox-gallery.js:646:14)

    at handler (src/core/data-structures/observable.js:78:6)

    at fire (src/gesture.js:437:17)

    at signalEmit (src/gesture-recognizers.js:400:9)

    at emit_ (src/gesture-recognizers.js:355:9)

    at acceptStart (src/gesture.js:536:15)

    at doPass_ (src/gesture.js:469:11)

    at afterEvent_ (src/gesture.js:299:9)

Notes
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`@erwinmombay` modified `src/core/data-structures/observable.js:78-78` in [#4249](https://github.com/ampproject/amphtml/pull/4249) (Sep 9, 2016)
`@rsimha` modified `src/gesture.js:437-439` in [#21212](https://github.com/ampproject/amphtml/pull/21212) (May 16, 2019)
`@rsimha` modified `src/gesture-recognizers.js:400-415` in [#21212](https://github.com/ampproject/amphtml/pull/21212) (May 16, 2019)
`@dvoytenko` modified `src/gesture-recognizers.js:353-369` in [#289](https://github.com/ampproject/amphtml/pull/289) (Sep 24, 2015)
`@dvoytenko` modified `src/gesture.js:528-598` in [#289](https://github.com/ampproject/amphtml/pull/289) (Sep 24, 2015)
`@dvoytenko` modified `src/gesture.js:466-478` in [#289](https://github.com/ampproject/amphtml/pull/289) (Sep 24, 2015)
`@dvoytenko` modified `src/gesture.js:282-308` in [#289](https://github.com/ampproject/amphtml/pull/289) (Sep 24, 2015)

**Seen in:**
- 05-15 Control (0310)
- 05-15 Nightly-Control (0310)
- 05-15 Stable (0310)
- 05-21 Experimental (0438+2)
- 05-28 Nightly (1634)
- +4 more

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

/cc @ampproject/release-on-duty