Use null-safe navigation for `denotify`

app/models/answer.rb

@@ -38,7 +38,7 @@ class Answer < ApplicationRecord
       comment.user&.decrement! :commented_count
       Subscription.denotify comment, self
     end
-    Notification.denotify self.question.user, self
+    Notification.denotify question&.user, self
     Subscription.destruct self
   end
   # rubocop:enable Rails/SkipsModelValidations

app/models/appendable/reaction.rb

@@ -9,7 +9,7 @@ class Appendable::Reaction < Appendable
   end
 
   before_destroy do
-    Notification.denotify parent.user, self
+    Notification.denotify parent&.user, self
     user&.decrement! :smiled_count
     parent&.decrement! :smile_count
   end