Have you tried using the AppInfo tool from Navisworks sample code? this wouls help you find out what is thowing the error. So far I have 2 guesses:
First, is your collection initialized? (new collection)
Second, only the RootItem has a value for the Model property: RootItem.Model; so if your item is a 1st level child of the RootItem, then you can access this property with Item.Parent.Model; but if your item is a N-th level children then you need to call the parent method N-times: Item.Parent.Parent.Parent.....Model.