Fix permission check with incorrect object type

closes #119

src/servala/core/rules.py

@@ -2,6 +2,10 @@ import rules
 
 
 def has_organization_role(user, org, roles):
+    from servala.core.models import Organization
+
+    if not isinstance(org, Organization):
+        return False
     memberships = org.memberships.all().filter(user=user)
     if roles:
         memberships = memberships.filter(role__in=roles)