When investigating T182465 (the failing browser tests) and proposed fix, I noticed a new issue impacting the "green-ness" of the tests.
Visit https://en.wikipedia.beta.wmflabs.org/wiki/Category:Page_Previews and hover over "Test previews no lead".
In the console this will throw an exception:
```
load.php?debug=false&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=vector&version=1b46sjy:52 jQuery.Deferred exception: Cannot read property 'replace' of undefined TypeError: Cannot read property 'replace' of undefined
at Object.o [as a] (<anonymous>:124:972)
at i (<anonymous>:127:857)
at o (<anonymous>:127:535)
at Object.<anonymous> (<anonymous>:126:988)
at mightThrow (https://en.wikipedia.beta.wmflabs.org/w/load.php?debug=false&lang=en&modules=jquery%2Cmediawiki%7Cmediawiki.legacy.wikibits&only=scripts&skin=vector&version=1b46sjy:49:598)
at process (https://en.wikipedia.beta.wmflabs.org/w/load.php?debug=false&lang=en&modules=jquery%2Cmediawiki%7Cmediawiki.legacy.wikibits&only=scripts&skin=vector&version=1b46sjy:50:269) undefined
jQuery.Deferred.exceptionHook @ load.php?debug=false&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=vector&version=1b46sjy:52
process @ load.php?debug=false&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=vector&version=1b46sjy:50
setTimeout (async)
(anonymous) @ load.php?debug=false&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=vector&version=1b46sjy:50
fire @ load.php?debug=false&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=vector&version=1b46sjy:46
fireWith @ load.php?debug=false&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=vector&version=1b46sjy:47
fire @ load.php?debug=false&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=vector&version=1b46sjy:47
fire @ load.php?debug=false&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=vector&version=1b46sjy:46
fireWith @ load.php?debug=false&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=vector&version=1b46sjy:47
done @ load.php?debug=false&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=vector&version=1b46sjy:126
(anonymous) @ load.php?debug=false&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=vector&version=1b46sjy:129
XMLHttpRequest.send (async)
send @ load.php?debug=false&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=vector&version=1b46sjy:130
ajax @ load.php?debug=false&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=vector&version=1b46sjy:125
jQuery.ajax @ load.php?debug=false&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=vector&version=1b46sjy:143
r @ VM77:126
c @ VM77:126
(anonymous) @ VM77:119
(anonymous) @ VM77:111
dispatch @ VM77:114
(anonymous) @ VM77:119
mightThrow @ load.php?debug=false&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=vector&version=1b46sjy:49
process @ load.php?debug=false&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=vector&version=1b46sjy:50
setTimeout (async)
(anonymous) @ load.php?debug=false&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=vector&version=1b46sjy:50
fire @ load.php?debug=false&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=vector&version=1b46sjy:46
fireWith @ load.php?debug=false&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=vector&version=1b46sjy:47
fire @ load.php?debug=false&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=vector&version=1b46sjy:47
fire @ load.php?debug=false&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=vector&version=1b46sjy:46
fireWith @ load.php?debug=false&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=vector&version=1b46sjy:47
deferred.(anonymous function) @ load.php?debug=false&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=vector&version=1b46sjy:51
(anonymous) @ VM77:146
setTimeout (async)
i @ VM77:146
(anonymous) @ VM77:119
(anonymous) @ VM77:111
(anonymous) @ VM77:115
(anonymous) @ VM77:130
dispatch @ load.php?debug=false&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=vector&version=1b46sjy:69
elemData.handle @ load.php?debug=false&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=vector&version=1b46sjy:66
```
Looking at the code:
https://github.com/wikimedia/mediawiki-extensions-Popups/blob/master/src/gateway/rest.js#L61
it appears to only know how to handle 404s and 200 responses. It should be adjusted.
Although this error is caught and swallowed, it does not display a page preview with "no summary available".