code: public function onUnregister(Registry $registry)
  	{
  		parent::onUnregister($registry);
  
  		// Unregister the fallback page
  		if ($this->fallback && $this->type == 'root' && $registry->isRegisteredAlias($this, 'contao.dns-fallback', $this->dns))
  		{
  			$registry->unregisterAlias($this, 'contao.dns-fallback', $this->dns);
  		}
  	}
docs: Unregister the contao.dns-fallback alias when the model is detached from the registry
  
  @param Registry $registry The model registry