In the script, the iframe uses a (at least to start out), I think it's a:
You need to change that to your blank https page. You didn't link to the script on DD here, or I could be more helpful:
Warning: Please include a link to the DD script in question in your post. See
this thread for the proper posting format when asking a question.
I did try your page but got no warning about non-secure content when activating the widget, perhaps you've fixed it but have an earlier version cached in your browser. Close all instances of the browser. Open it and delete all files (clear the cache), then try loading your page and activating the widget.
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