`open-all-notifications` throws error on `github.com/*/*/notifications` #2397

(Firefox 68.0.2 (64-bit) on Windows 10, plugin version 19.9.2)

Everything is fine on https://github.com/notifications:

But if I navigate to the notifications of a specific repo, e.g. https://github.com/prisma/nexus-prisma/notifications, I get a TypeError:

 TypeError: "button is null"
    groupButtons moz-extension://57b5be31-7fbe-4b7d-bae1-b2f5de44e6b4/content.js:4271
    addOpenAllButton moz-extension://57b5be31-7fbe-4b7d-bae1-b2f5de44e6b4/content.js:4342
    open_all_notifications_update moz-extension://57b5be31-7fbe-4b7d-bae1-b2f5de44e6b4/content.js:4350
    open_all_notifications_init moz-extension://57b5be31-7fbe-4b7d-bae1-b2f5de44e6b4/content.js:4356
    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

I don't know if this actually breaks anything, but I guess it should be fixed anyway.