Warnings raised in AMP_Style_Sanitizer::get_validate_response_data() due to nodes no longer existing

[westonruter]

Bug Description

When activating this gist plugin which is rendering an amp-story without the required standalone attribute (resulting in the entire amp-story being removed), there are warnings being raised by code in AMP_Style_Sanitizer::get_validate_response_data():

PHP Warning:  Couldn't fetch DOMElement. Node no longer exists in …/amp/includes/sanitizers/class-amp-style-sanitizer.php on line 2924
PHP Notice:  Undefined property: DOMElement::$attributes in …/amp/includes/sanitizers/class-amp-style-sanitizer.php on line 2924
PHP Warning:  Invalid argument supplied for foreach() in …/amp/includes/sanitizers/class-amp-style-sanitizer.php on line 2924
PHP Warning:  Couldn't fetch DOMElement. Node no longer exists in …/amp/includes/sanitizers/class-amp-style-sanitizer.php on line 2928
PHP Notice:  Undefined property: DOMElement::$nodeName in …/amp/includes/sanitizers/class-amp-style-sanitizer.php on line 2928

Line 2924:

amp-wp/includes/sanitizers/class-amp-style-sanitizer.php

Line 2924 in f158192

foreach ( $pending_stylesheet['element']->attributes as $attribute ) {

Line 2928:

amp-wp/includes/sanitizers/class-amp-style-sanitizer.php

Line 2928 in f158192

'name' => $pending_stylesheet['element']->nodeName,

This is strange to me because the original DOMElement object is being stored in the pending_stylesheets array. Somehow the reference is going bad.

Expected Behaviour

No warnings should be emitted when the style sanitizer is iterating over the pending_stylesheets when elements are removed from the document.

Steps to reproduce

0. Enable AMP on all templates (e.g. in Standard mode)
1. Activate the gist plugin
2. Access a URL that would normally be an AMP page.
3. Now add ?try_node_no_longer_exists_warning=1
4. Click Validate URL in the admin bar.
5. See warnings in the error log.

Screenshots

In the Stylesheets metabox you'll also see:

The title attribute value is <>.

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Acceptance criteria

Implementation brief

QA testing instructions

Demo

Changelog entry

[westonruter]

Related: #3393.