FIX: do not error when private message is sent (PR #41)

Bug was introduced here FIX: Remove custom fields from unapproved posts when user is removed … · discourse/discourse-category-experts@dd7fbd2 · GitHub

When private message is sent there is no category and this code was crashing

GitHub

      category = post.topic&.category
      return if !category

      group_id = user.expert_group_ids_for_category(category)&.first