Revoke incorrect fix: should fix Preprint.save() instead

Test: test_preprint_contributor_signal_sent_on_creation

osf/models/mixins.py

@@ -1383,8 +1383,11 @@ def add_contributor(self, contributor, permissions=None, visible=True,
                 )
             if save:
                 self.save()
-            Preprint = apps.get_model('osf.Preprint')
-            if (self._id and contrib_to_add) or (isinstance(self, Preprint) and contrib_to_add):
+            if self._id and contrib_to_add:
+            # TODO: remove the comment after `Preprint.save()` has been fixed.
+            # There is a bug for Preprint when the first `.save()` is called. It triggers a series of events which
+            # includes adding creator as the contributor. However, with versioned guid, there is a gap between preprint
+            # is saved and guid is updated, during which `self._id` is None and the signal can not be be sent.
                 project_signals.contributor_added.send(self,
                                                        contributor=contributor,
                                                        auth=auth, email_template=send_email, permissions=permissions)