(Firefox 68.0.2 (64-bit) on Windows 10, plugin version 19.9.2)
Happens for me on for example this PR: https://github.com/fastlane/fastlane/pull/15255

19:36:52.676 โ wait-for-build content.js:2837:15
19:36:52.702
TypeError: "select_dom_default.a.last(...) is undefined"
getLastCommit moz-extension://57b5be31-7fbe-4b7d-bae1-b2f5de44e6b4/content.js:5401
addEventListener moz-extension://57b5be31-7fbe-4b7d-bae1-b2f5de44e6b4/content.js:5436
wait_for_build_init moz-extension://57b5be31-7fbe-4b7d-bae1-b2f5de44e6b4/content.js:5519
features_run moz-extension://57b5be31-7fbe-4b7d-bae1-b2f5de44e6b4/content.js:2833
add moz-extension://57b5be31-7fbe-4b7d-bae1-b2f5de44e6b4/content.js:2880
onAjaxedPages moz-extension://57b5be31-7fbe-4b7d-bae1-b2f5de44e6b4/content.js:2795
onAjaxedPagesRaw moz-extension://57b5be31-7fbe-4b7d-bae1-b2f5de44e6b4/content.js:2790
content.js:2838:15
features_run moz-extension://57b5be31-7fbe-4b7d-bae1-b2f5de44e6b4/content.js:2838
add moz-extension://57b5be31-7fbe-4b7d-bae1-b2f5de44e6b4/content.js:2880
onAjaxedPages moz-extension://57b5be31-7fbe-4b7d-bae1-b2f5de44e6b4/content.js:2795
onAjaxedPagesRaw moz-extension://57b5be31-7fbe-4b7d-bae1-b2f5de44e6b4/content.js:2790
I don't know if this actually breaks anything, but I guess it should be fixed anyway.
Same on Chrome:

I cannot confirm this. Is there any other extension interfering?
No, I tested with only Refined Github enabled.
I can only reproduce on repos where I am admin or can merge PRs (feature probably only runs there).
Any way I can help debugging this?
This seems to be the code failing here:
https://github.com/sindresorhus/refined-github/blob/be74449658e54b01397eca2766d8aab3ac0a54ce/source/libs/pr-ci-status.ts#L7-L9
This is the full HTML code of the page as it is shown to me:
https://pastebin.com/Ry8HbgF2
This is the only occurence of commit-id:
<div class="text-right">
<code>
<a href="/fastlane/fastlane/pull/15255/commits/e27e62e21686e4a9ae2962a8d0589c9f41aaf26b" class="commit-id">e27e62e</a>
</code>
</div>
commits-pushed does not appear at all, but if I understand the code snippet correctly that is exactly what _should_ not appear for a match, correct?
I can only reproduce on repos where I am admin or can merge PRs (feature probably only runs there).
I still can't confirm this error. For example https://github.com/jerone/PackageSize/pull/2, this is an PR and I'm admin of this repo. Checkbox is visible and no errors in console.
commits-pusheddoes not appear at all, but if I understand the code snippet correctly that is exactly what _should_ not appear for a match, correct?
I'm not familiar with this feature, so I'll let @fregante answer your questions.
What do you get in the console when you execute the following snippet $$('[id^="commits-pushed"] .commit-id');?
New unclosed PR where I get this error:
https://github.com/fastlane/fastlane/pull/15269
What do you get in the console when you execute the following snippet
$$('[id^="commits-pushed"] .commit-id');?
22:54:17.015 $$('[id^="commits-pushed"] .commit-id');
22:54:17.026 Array []

Ah yes, I can confirm. It needs to be an unclosed PR.
That selector probably needs to be changed to select.last('.js-commit-group-commits .commit-id').
Most helpful comment
Ah yes, I can confirm. It needs to be an unclosed PR.
That selector probably needs to be changed to
select.last('.js-commit-group-commits .commit-id').