Ninja Forms with ReCaptcha not loading in pop up

Hi, a client added Google ReCaptcha to some Ninja Forms on their website which are now not loading when they’re inserted into a popup.

The site tested fine without the ReCaptcha when it was in development, so fairly confident it’s causing some sort of conflict. In Chrome inspector Console you can see this JS error:

Uncaught Error: ReCAPTCHA placeholder element must be empty
    at Object.or [as render] (recaptcha__en.js:378)
    at HTMLDivElement.<anonymous> (front-end-deps.js?ver=3.2.3:129)
    at Function.each (jquery.js:2)
    at a.fn.init.each (jquery.js:2)
    at n.renderCaptcha (front-end-deps.js?ver=3.2.3:120)
    at _ (backbone.min.js?ver=1.2.3:1)
    at m (backbone.min.js?ver=1.2.3:1)
    at f (backbone.min.js?ver=1.2.3:1)
    at u.Channel.u.trigger (backbone.min.js?ver=1.2.3:1)
    at front-end.js:3

What’s really strange is that in testing, I found that some popups would not show any Ninja form, even one that was loading onto the same page in a different popup! This was my workaround solution on the test server, which ‘fixed’ the issue but doesn’t seem very stable as a fix.

Let me know if you want any further details, Janto

The page I need help with: [log in to see the link]