code: public function getUrl($url)
      {
          if (($baseUrl = $this->getBaseUrl()) !== null) {
              return $baseUrl . '/' . ltrim($url, '/');
          }
  
          throw new InvalidConfigException('The "baseUrl" property must be set.');
      }
docs: Converts a relative URL into an absolute URL using [[baseUrl]].
  @param string $url the relative URL to be converted.
  @return string the absolute URL
  @throws InvalidConfigException if [[baseUrl]] is not set