Fix NPE in cache control decorator

util/cache.py

@@ -29,6 +29,7 @@ def no_cache(f):
   @wraps(f)
   def add_no_cache(*args, **kwargs):
     response = f(*args, **kwargs)
+    if response is not None:
       response.headers['Cache-Control'] = 'no-cache, no-store, must-revalidate'
     return response
   return add_no_cache