Fixed #24418 -- Prevented crash in refresh_from_db with null fk

Thanks Johannes Lerch for the report, Tim Graham for the test case,
and Simon Charette for the review.

1 files changed, 1 insertions, 1 deletions

diff --git a/django/db/models/base.py b/django/db/models/base.py
index a505efa87d..cb1d37f9df 100644
--- a/django/db/models/base.py
+++ b/django/db/models/base.py
@@ -596,7 +596,7 @@ class Model(six.with_metaclass(ModelBase)):
             if field.rel and field.get_cache_name() in self.__dict__:
                 rel_instance = getattr(self, field.get_cache_name())
                 local_val = getattr(db_instance, field.attname)
-                related_val = getattr(rel_instance, field.related_field.attname)
+                related_val = None if rel_instance is None else getattr(rel_instance, field.related_field.attname)
                 if local_val != related_val:
                     del self.__dict__[field.get_cache_name()]
         self._state.db = db_instance._state.db