It is possible a <ref> contains no visible content, e.g. only whitespace or HTML elements that are all invisible (display: none). Example: https://de.wikipedia.org/wiki/Adelsgesellschaft.
The Reference Previews popup appears empty in this case. This looks like a bug in the Popups extension, even if it was a deliberate choice made by the article author.
Since the only useful element in an empty popup is the "jump down to reference" link, this is what an empty popup should do instead: The popup should not be shown, but immediately jump down to the link target.