PERF: Fix broken memoization in `GroupShowSerializer`.

PERF: Fix broken memoization in GroupShowSerializer.

diff --git a/app/serializers/group_show_serializer.rb b/app/serializers/group_show_serializer.rb
index 17e11ac..29a81c6 100644
--- a/app/serializers/group_show_serializer.rb
+++ b/app/serializers/group_show_serializer.rb
@@ -58,6 +58,7 @@ class GroupShowSerializer < BasicGroupSerializer
   end
 
   def fetch_group_user
-    @group_user ||= object.group_users.find_by(user: scope.user)
+    return @group_user if defined?(@group_user)
+    @group_user = object.group_users.find_by(user: scope.user)
   end
 end

GitHub sha: 468417a7