The pop-up occurs on the main, open-to-the public page of the website, before any data that could reasonably be expected to require https is requested or submitted.
The website is required for use by many people in the course of doing their jobs. It is inappropriate to interrupt or delay people’s work because a website contains something that could be, but has not been, exploited for dangerous purposes. As I’ve said before, Avast should not be blocking websites merely because they “might” contain something bad, but only if they actually DO contain something bad.