[1.8.x] Fixed #25040 -- Fixed migrations state crash with GenericForeignKey

Backport of 60f795c060 from master

django/db/migrations/state.py

@@ -45,7 +45,7 @@ def get_related_models_recursive(model):
     def _related_models(m):
         return [
             f.related_model for f in m._meta.get_fields(include_parents=True, include_hidden=True)
-            if f.is_relation and not isinstance(f.related_model, six.string_types)
+            if f.is_relation and f.related_model is not None and not isinstance(f.related_model, six.string_types)
         ] + [
             subclass for subclass in m.__subclasses__()
             if issubclass(subclass, models.Model)

docs/releases/1.8.5.txt

@@ -11,3 +11,5 @@ Bugfixes
 
 * Fixed ``AssertionError`` in some delete queries with a model containing a
   field that is both a foreign and primary key (:ticket:`24951`).
+
+* Fixed a migrations crash with ``GenericForeignKey`` (:ticket:`25040`).

tests/migrations/test_state.py

@@ -1,4 +1,5 @@
 from django.apps.registry import Apps
+from django.contrib.contenttypes.fields import GenericForeignKey
 from django.db import models
 from django.db.migrations.operations import (
     AddField, AlterField, DeleteModel, RemoveField,
@@ -976,6 +977,12 @@ class RelatedModelsTests(SimpleTestCase):
         self.assertRelated(S, [A, B, T])
         self.assertRelated(T, [A, B, S])
 
+    def test_generic_fk(self):
+        A = self.create_model("A", foreign_keys=[models.ForeignKey('B'), GenericForeignKey()])
+        B = self.create_model("B", foreign_keys=[models.ForeignKey('C')])
+        self.assertRelated(A, [B])
+        self.assertRelated(B, [A])
+
     def test_abstract_base(self):
         A = self.create_model("A", abstract=True)
         B = self.create_model("B", bases=(A,))