Hi, yes, it's a kind of a bug in that we should have disabled the entry under Text Content tab in the "lightbox" mode in v3.6.4 as it's not supported there.
In the latest v3.6.5 beta, this has been drastically reworked, and so when there's no image under the Image Content tab on the Hotspot form in the "lightbox" mode, there's now some very comprehensive support implemented, including:
1) external "embeds" of any kind (e.g youtube video), by simply pasting a URL under Text Content.
2) full responsive popup layout / sizing with control over relative width, height, max width/height, and an option to show the popup centered in the browser window instead (i.e outside of the viewer container).
3) customizable close button in the right top corner.
4) scrollable content in the popup.
5) and a couple of other extras.
If you are on Windows, we can send you a new build with the features listed above. It may take a week or so to iron out the Mac version though