Check if `system` is truthy in `Note.cross_reference?`

app/models/note.rb

@@ -99,7 +99,7 @@ class Note < ActiveRecord::Base
   end
 
   def cross_reference?
-    SystemNoteService.cross_reference?(note)
+    system && SystemNoteService.cross_reference?(note)
   end
 
   def max_attachment_size