Google reCaptcha response “Uncaught (in promise) null”

Turns out it also occurs when a site is not “registered” in the Google recaptcha/admin Domains area.

Solution: Add the domain in the recaptcha admin area:

1. Sign into your Google account where your recaptcha keys are registered
2. Type into Google “google recpatcha admin console”
3. Go to your settings for your (production) key
4. In “Domains”, add these two entries:

localhost
127.0.0.1

5. Save it and test your recaptcha.

I made this error when I switched from a development key to a production key. The production key did not have any entries for localhost.

I configured the API response to sit behind a proxy-redirect. Therefore the verification was working in a localhost environment which was not configured in the Google Admin console which caused this generic error.

Credit to @Christian Žagarskas who pointed it out in his comment.