fix deletion of inbox entries when deleting an user

app/models/inbox.rb

@@ -22,7 +22,8 @@ class Inbox < ApplicationRecord
   end
 
   after_destroy do
-    user.touch(:inbox_updated_at)
+    # user might not exist at this point (account deleted, records are cleaned up async)
+    user&.touch(:inbox_updated_at)
   end
 
   def answer(answer_content, user)

spec/models/inbox_spec.rb

@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+describe Inbox, type: :model do
+  describe "associations" do
+    it { should belong_to(:user) }
+    it { should belong_to(:question) }
+  end
+
+  describe "before_destroy" do
+    let(:user) { FactoryBot.create(:user) }
+    let(:question) { FactoryBot.create(:question, author_is_anonymous: true) }
+
+    it "does not fail if the user wants to delete their account" do
+      Inbox.create(user:, question:)
+
+      # this deletes the User record and enqueues the deletion of all
+      # associated records in sidekiq
+      user.destroy!
+
+      # so let's drain the queues
+      expect { Sidekiq::Worker.drain_all }.not_to raise_error
+    end
+  end
+end