Steps to reproduce the issue

• Install the testing sampledata plugin (or any other one) from https://github.com/joomla-extensions/testing-sample-data/releases/tag/v3.8.0. That one is expected to be broken in 4.0 but for this test it will do.
• Try installing the "Blog Sample Data" from the Control Panel.

Expected result

Installs fine

Actual result

It doesn't install. A JS error shows in the browser

Data response looks like this:

Please note that data is an array and the first entry is null (coming from the "testing" plugin returning empty). Our JS apparently checks the first entry and doesn't find a success property.

I'm pinging @C-Lodder here as he rewrote the JS lately. I honestly don't know if that bug existed before that rewrite or not.