gracefully handle missing model classes in Page.specific()

wagtail/wagtailcore/models.py

@@ -425,7 +425,15 @@ class Page(six.with_metaclass(PageBase, MP_Node, ClusterableModel, index.Indexed
         # the ContentType.objects manager keeps a cache, so this should potentially
         # avoid a database lookup over doing self.content_type. I think.
         content_type = ContentType.objects.get_for_id(self.content_type_id)
-        if isinstance(self, content_type.model_class()):
+        model_class = content_type.model_class()
+        if model_class is None:
+            # Cannot locate a model class for this content type. This might happen
+            # if the codebase and database are out of sync (e.g. the model exists
+            # on a different git branch and we haven't rolled back migrations before
+            # switching branches); if so, the best we can do is return the page
+            # unchanged.
+            return self
+        elif isinstance(self, model_class):
             # self is already the an instance of the most specific class
             return self
         else: