As the notification says, patch tester support on 4.0 is experimental at best right now. It isn't feasible to keep it in sync with 4.0 while it's still undergoing changes (actually a proper 4.0 layout wasn't even added to the code until after the last beta was tagged, and that doesn't yet include JavaScript changes that are required).

It seems someone has busted the JavaScript in core.js. Or there's some other B/C break with the new JavaScript and/or template that has to be reverse engineered since right now there is zero documentation on the changes.

Uncaught TypeError: Cannot read property 'joomla.jtext' of null
    at Object.Joomla.getOptions (core.js?1fef521…:129)
    at Object._ (core.js?1fef521…:74)
    at Object.error (fetcher.js?1fef521…:81)
    at fire (jquery.js?1fef521…:3305)
    at Object.fireWith [as rejectWith] (jquery.js?1fef521…:3435)
    at done (jquery.js?1fef521…:9244)
    at XMLHttpRequest.<anonymous> (jquery.js?1fef521…:9484)

thanks @mbabker

@C-Lodder @dgt41 can you help here with a JavaScript migration guide for 4.0?

@franz-wohlkoenig i think we should keep this open as this is still a issue even if the 4.0 support is experimental ?

The experimental warning is really just more to say "don't expect everything to work right all the time", not "it's 4.0, no bug fixes ever".

Turns out this wasn't a JavaScript error, rather a minor B/C break between JHttpResponse in 3.x and Joomla\Http\Response in 4.0. This is fixed, though I could still use help with the JavaScript migration.