Return None, instead of raising AttributeError on MarkupTextDescripto…

…r if no instance (#66)

Django 1.11.2 get_or_create object is passing in None instance

machina/models/fields.py

@@ -110,7 +110,7 @@ def __init__(self, field):
 
     def __get__(self, instance, owner):
         if instance is None:
-            raise AttributeError(_('Can only be accessed via an instance.'))
+            return None
         raw = instance.__dict__[self.field.name]
         if raw is None:
             return None

tests/unit/forum/test_models.py

@@ -113,3 +113,9 @@ def test_can_send_a_specific_signal_when_a_forum_is_moved(self):
             self.top_level_forum.parent = self.top_level_cat
             self.top_level_forum.save()
             assert receiver.call_count == 1
+
+    def test_get_or_create(self):
+        forum, created = Forum.objects.get_or_create(name="Test Forum", type=0)
+        assert created is True
+        assert isinstance(forum, Forum)
+        assert forum.name == "Test Forum"