Handle the case in is_downref() where the src file doesn't have a std_level or intended_std_level set.

 - Legacy-Id: 7198

Author: levkowetz

ietf/doc/models.py

@@ -176,8 +176,10 @@ def is_downref(self):
 
         if self.source.get_state().slug == 'rfc':
             source_lvl = self.source.std_level.slug
-        else:
+        elif self.source.intended_std_level:
             source_lvl = self.source.intended_std_level.slug
+        else:
+            source_lvl = None
 
         if source_lvl not in ['bcp','ps','ds','std']:
             return None