If noteable is nil - make discussion outdated

app/models/note.rb

@@ -296,6 +296,7 @@ class Note < ActiveRecord::Base
   # If not - its outdated diff
   def active?
     return true unless self.diff
+    return false unless noteable
 
     noteable.diffs.each do |mr_diff|
       next unless mr_diff.new_path == self.diff.new_path