app/controllers/thredded/private_post_previews_controller.rb
def preview
      @private_post = Thredded::PrivatePost.new(private_post_params)
      @private_post.postable = Thredded::PrivateTopic.friendly_find!(params[:private_topic_id])
      authorize @private_post, :create?
      render_preview
    end
def preview
      @private_post = Thredded::PrivatePost.new(private_post_params)
      @private_post.postable = Thredded::PrivateTopic.friendly_find!(params[:private_topic_id])
      authorize @private_post, :create?
      render_preview
    end
Preview a new post