@Anonymous, see this helpful explanation by @Jon.Balgley from this thread:
if the DrawingView is not associative with the DesignViewRepresentation (DVR), then subsequently calling DrawingView.ActiveDesignViewRepresentation will return the empty string.
I can give a halfway-good justification for that: Once you do myDrawingView.SetDesignViewRepresentation("myDVR", False) [or the "associative" box becomes unchecked by any other method], then [the drawing view] is using a "snapshot" of the state of the DVR; it's not really using the DVR after that point. After all, you can turn components on/off in the drawing, independently of their visibility in the DVR in the assembly. So subsequently asking for the "active" DVR of the drawing-view is no longer accurate. I think you're asking for "what was the last DVR assigned to this drawing view?" ... not the "active" one. That would be a new API property. Is that what you want? If so, you could enter that in the "Inventor Ideas" forum (or reply here and I can do it).
(Bracketed additions by me)
There is a feature request for this that you can vote for (please do):
API improvement: Reassociate a Drawing View with its Design View Representation