On "quit-and-save" pedigree complains that a family has no members if all members are unlinked

To reproduce:

• create a family, create a pedigree, add some members, save
• un-link ALL members, do any other modifications (e.g. add some nodes)
• click close, pick "save"
• expected: pedigree is saved and closed
  -> actual: the "There are no patients linked to this pedigree and thus family has no members." dialog pops up