anchor.diff:        if (!PinUtility.isUuid(uuid)) {
           throw new Exception('[CartAction] handlePostMarkdownFetch: Invalid post uuid: ${uuid}');
         }
  -      CartPost post = CartModel.instance.postList.find(uuid);
  +      post = CartModel.instance.postList.find(uuid);
         if (post == null) {
           return new Future.value(null); // post not exists, null return
         } else {
  
positive.code:     if (!CartSystem.instance.actionPreProcess(ctx)) {
        return;
      }
      if (!_filterIsMaster(ctx)) {
        return;
      }
  
      String uuid;
      File mdFile;
  
      HttpBodyHandler.processRequest(ctx.req)
  
negative.code:     })
      .then((_) {
        if (_ == null) {
          return new Future.value(''); // post not exists, '' return
        } else {
          if (_ == false) { // post exists, markdown file not exists, error
            throw new Exception('[CartAction] handlePostMarkdownFetch: Target markdown file of the post not found: ${uuid}');
  