[1.11.x] Refs #27148 -- Fixed UUIDField.to_python(None) crash.

Regression in 2f9861d823.

Backport of d26413113c from master

django/db/models/fields/__init__.py

@@ -2387,7 +2387,7 @@ class UUIDField(Field):
         return value.hex
 
     def to_python(self, value):
-        if not isinstance(value, uuid.UUID):
+        if value is not None and not isinstance(value, uuid.UUID):
             try:
                 return uuid.UUID(value)
             except (AttributeError, ValueError):

tests/model_fields/test_uuid.py

@@ -54,13 +54,16 @@ class TestSaveLoad(TestCase):
             UUIDModel.objects.create(field='not-a-uuid')
 
 
-class TestMigrations(SimpleTestCase):
+class TestMethods(SimpleTestCase):
 
     def test_deconstruct(self):
         field = models.UUIDField()
         name, path, args, kwargs = field.deconstruct()
         self.assertEqual(kwargs, {})
 
+    def test_to_python(self):
+        self.assertIsNone(models.UUIDField().to_python(None))
+
 
 class TestQuerying(TestCase):
     def setUp(self):
@@ -88,6 +91,10 @@ class TestSerialization(SimpleTestCase):
         '[{"fields": {"field": "550e8400-e29b-41d4-a716-446655440000"}, '
         '"model": "model_fields.uuidmodel", "pk": null}]'
     )
+    nullable_test_data = (
+        '[{"fields": {"field": null}, '
+        '"model": "model_fields.nullableuuidmodel", "pk": null}]'
+    )
 
     def test_dumping(self):
         instance = UUIDModel(field=uuid.UUID('550e8400e29b41d4a716446655440000'))
@@ -98,6 +105,10 @@ class TestSerialization(SimpleTestCase):
         instance = list(serializers.deserialize('json', self.test_data))[0].object
         self.assertEqual(instance.field, uuid.UUID('550e8400-e29b-41d4-a716-446655440000'))
 
+    def test_nullable_loading(self):
+        instance = list(serializers.deserialize('json', self.nullable_test_data))[0].object
+        self.assertIsNone(instance.field)
+
 
 class TestValidation(SimpleTestCase):
     def test_invalid_uuid(self):