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

[ampprojectbot]

Details

Error report: link
First seen: Mar 14, 2021
Frequency: ~ 6,648/day

Stacktrace

Error: null is not an object (evaluating 'a.getElementsByTagName')
    at getElementsByTagName (extensions/amp-live-list/0.1/live-list-manager.js:187:8)

Notes

@rsimha modified extensions/amp-live-list/0.1/live-list-manager.js:187-188 in #21212 (May 16, 2019)

Seen in:

• 11-24 Control (2025+1)
• 11-24 Nightly-Control (2025+1)
• 11-24 Stable (2025+1)

/cc @ampproject/release-on-duty

[rcebulko]

This error occurs in getLiveLists_(doc) when doc == null. This is called only by updateLiveLists_(doc), which in turn is called only in a .then( callback after fetchDocument (see https://github.com/ampproject/amphtml/blame/711d5de27696965cdfb1d9a6899a6ac146134410/extensions/amp-live-list/0.1/live-list-manager.js#L148). Even though the JSDoc comments indicate fetchDocument (see https://github.com/ampproject/amphtml/blob/main/src/document-fetcher.js#L24) should always return a Document, it's possible for xhr.responseXML to be null if the XHR is unsuccessful, blocked, or can't be parsed.

@ampproject/wg-performance we should add a userAssert(xhr.responseXML) with a more meaningful message to indicate what request failed and why, rather than forwarding a null along for calling code to handle. We should reject, not resolve, in this case.

@samouri This is an error that would have been caught if we were a few months into the future on type-checking 🥳