code: public function getCustomField($class, $slug = null)
      {
          if (!$slug) {
              throw CustomFieldsHelperException::slugIsMissing();
          }
          
          $resolveClass = (is_string($class)) ? $class : get_class($class);
          if (!$this->cache[$resolveClass][$slug]) {
              $this->_cacheCustomFields($resolveClass);
          }
          
          return $this->cache[$resolveClass][$slug];
      }
docs: Return a requested customField
  
  @param object|string $class The requested class
  @param string $slug The slug. BEWARE If the slug is null, throw a proper CustomFieldsHelperException
  @return CustomField The requested CustomField
  @throws CustomFieldsHelperException if $slug is null