Fix crash when a notification is deleted while a bundle is being sent

app/mailers/notification/bundle_mailer.rb

@@ -7,10 +7,14 @@ class Notification::BundleMailer < ApplicationMailer
     @user = bundle.user
     @bundle = bundle
     @notifications = bundle.notifications
+      .preload(:card, :creator, source: [ :board, :creator ])
+      .reject { |n| n.source.nil? || n.card.nil? }
     @unsubscribe_token = @user.generate_token_for(:unsubscribe)
 
+    if @notifications.any?
       mail \
         to: bundle.user.identity.email_address,
         subject: "Fizzy#{ " (#{ Current.account.name })" if @user.identity.accounts.many? }: New notifications"
     end
+  end
 end

test/mailers/notification/bundle_mailer_test.rb

@@ -97,6 +97,15 @@ class Notification::BundleMailerTest < ActionMailer::TestCase
     assert_equal "#1 Fix the <code>bug</code> in production", title_link.inner_html
   end
 
+  test "skips notifications whose source event was deleted" do
+    notification = create_notification(@user)
+    notification.source.destroy
+
+    email = Notification::BundleMailer.notification(@bundle)
+    assert_not email.respond_to?(:deliver) && email.message.is_a?(Mail::Message),
+      "Should not generate a real email when all notifications are stale"
+  end
+
   private
     def create_notification(user, source: events(:logo_published))
       Notification.create!(user: user, creator: user, source: source, created_at: 30.minutes.ago)