Fix for GroupTagKeySerializer. (#6006)

Author: bretthoerner

src/sentry/api/serializers/models/grouptagkey.py

@@ -13,7 +13,7 @@ def get_attrs(self, item_list, user):
             t.key: t.get_label()
             for t in
             TagKey.objects.filter(
-                project_id=item_list[0].project.id, key__in=[
+                project_id=item_list[0].project_id, key__in=[
                     i.key for i in item_list])
         }