code: public function get($id)
      {
          if (isset($this->singletons[$id])) {
              return $this->singletons[$id];
          }
  
          $methodName = 'get' . Util::mapIdToCamelCase($id);
  
          if (method_exists($this, $methodName)) {
              return $this->$methodName();
          }
  
          throw new \InvalidArgumentException("Unknown service '$id' in container '" . get_called_class() . "'");
      }
docs: Try to get the service for the given id. If not found an exception is thrown.
  
  @param string $id
  
  @return mixed
  @throws \InvalidArgumentException when the service is not present in the container