avoid 502 on creating preprint version from rejected version

Author: mkovalua

osf/models/preprint.py

@@ -407,7 +407,7 @@ def check_unfinished_or_unpublished_version(self):
         """Check and return the "initiated but unfinished version" and "unfinished or unpublished version".
         """
         last_not_rejected_version = self.get_last_not_rejected_version()
-        if last_not_rejected_version.date_published:
+        if not last_not_rejected_version or last_not_rejected_version.date_published:
             return None, None
         if last_not_rejected_version.machine_state == 'initial':
             return last_not_rejected_version, None