Plugin: MyPopUps Center Alignment Issue

This issue is regarding the Popup alignment which doesn’t stay in the middle of the screen. Although it does center, it just stays at the very top of the page, rather than the middle.
This is the website where you create the popup: https://mypopups.com/

My apologies for not sending a link to the issue, but my client didn’t have time for this to mess up, so I had to use an alternative method.

I contacted support for this plugin and they know of it to be an issue with WordPress, so they asked if I could mention the issue on the WordPress forum. So here it is!

Any help is appreciated ??

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