I finished There Stands the Grass before doing this quest and now I can't deliver the package to Dr. Hildern. The only conversation option for him is goodbye.

I can't recruit Cass without finishing this quest.

Signed up to say I've just ran into this bug (and to see if there was a work around). How annoying. I hope the next patch fixes this and it's blocking a number of missions as well as recruiting Cass as a companian!