You've probably overcome or worked around this issue by now but not a lot has changed two years on. I too had this issue and after a fair few hours trying to sort it out I found out that I didn't have an EmptyDataTemplate in my FormView. I added <EmptyDataTemplate></EmptyDataTemplate> inside my FormView and that fixed it for me!
PS This is my first forum post for many years - I registered on the site especially because I almost gave up hope resolving this myself and I don't want others to suffer like I did 