fix the build

fix the build

diff --git a/app/controllers/list_controller.rb b/app/controllers/list_controller.rb
index 17fcccd..f05e1a4 100644
--- a/app/controllers/list_controller.rb
+++ b/app/controllers/list_controller.rb
@@ -440,7 +440,7 @@ class ListController < ApplicationController
       return period if top_topics.count == SiteSetting.topics_per_period_in_top_page
     end
 
-    false
+    nil
   end
 
   def self.best_periods_for(date, default_period = :all)

GitHub sha: d7ff6407