I haven't seen any code that initializes this.htmlNode yet, so it could be undefined here.

I don't understand the hierarchy here. Looks like you're making thumbnails grandchildren of this display group instead of children.

Oh, I think I see. lastElementChild is the ul elements for this group.  I think you should make the div and the ul element properties of the object.  So in addition to (or instead of) this.htmlNode, you'd have this.headerElement and this.listContainer or something like that. That would make the code easier to understand.