docs: function renderToolbarItems (req, res, modelName, cb) {
  
      if (typeof modelName !== 'string' || !modelName.length) {
          throw new Error('modelName is required and must be of type String and cannot be empty.');
      }
  
      linz.api.model.list(req, modelName, function (err, list) {
  
          if (err) {
              return cb(err);
          }
  
          if (!list.toolbarItems.length) {
              return cb(null);
          }
  
          async.each(list.toolbarItems, function (item, itemDone) {
  
              if (!item.renderer) {
                  return itemDone(new Error('Renderer function is missing for toolbar item: ' + require('util').inspect(item) + '. Check your \'' + modelName + '\' model for more details.'));
              }
  
              // this is a custom action, let's execute the function to get the HTML markup
              item.renderer(req, res, function (itemErr, html) {
  
                  if (itemErr) {
                      return itemDone(itemErr);
                  }
  
                  item.html = html;
  
                  return itemDone(null);
              });
  
          }, function (itemErr) {
              return cb(itemErr, list.toolbarItems);
          });
  
      });
  
  }