Blank Pop-up window showing on front page of my blog

I’ve got a blank pop-up window displaying when I go on the blog and every time I click on a new article or page.
Before this, I had a problem with my sidebar going to the bottom of the page, and after that suddenly went away, this blank pop-up window came up. Not sure if they’re related.
I’ve never used pop-up windows before and it seems really random. I don’t mess around with css and try not to download too many plugins. Any help would be much appreciated!

My blog; https://www.hardcoreheroines.co.uk