[PopUp Pro] insecure image in wp-admin

Our site is in SSL and we've the latest PopUp Pro 4.8.1 installed. It load an image insecurely in wp-admin.

Mixed Content: The page at 'https://domain.com/wp-admin/media-new.php' was loaded over HTTPS, but requested an insecure image 'http://domain.com/wp-content/plugins/popover/img/icon.png'. This content should also be served over HTTPS.