code: func (m *Manager) LookupRuntimeHandler(runtimeClassName *string) (string, error) {
  	if runtimeClassName == nil || *runtimeClassName == "" {
  		// The default RuntimeClass always resolves to the empty runtime handler.
  		return "", nil
  	}
  
  	name := *runtimeClassName
  
  	rc, err := m.lister.Get(name)
  	if err != nil {
  		if errors.IsNotFound(err) {
  			return "", err
  		}
  		return "", fmt.Errorf("Failed to lookup RuntimeClass %s: %v", name, err)
  	}
  
  	return rc.Handler, nil
  }
docs: // LookupRuntimeHandler returns the RuntimeHandler string associated with the given RuntimeClass
  // name (or the default of "" for nil). If the RuntimeClass is not found, it returns an
  // errors.NotFound error.