I’m confused a little bit, you are talking about WordPress 4.5 but your website runs on 4.8, also Ninja forms have updated their plugin to run on 4.8.

I’m not getting any errors on my chrome developer console from the page you link I see a select input, that gets another selection input (for example if I select But, another one pops out ) after submitting it though it just reloads the page.

I don’t know what it should be doing or if that is the problem.

But mainly, you have WordPress 4.8 and if you are running the latest Ninja Forms it should support 4.8, that’s what their plugin is saying.

jQuery is loaded through WordPress except if the theme developer of Mega thought of changing that which would be a disaster either way.

Please explain further what the actual problem is so we can understand at least what you are trying to achieve here and help if possible.

Best regards,
Konstantinos