core-bundle/src/Resources/contao/models/PageModel.php
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);
		}
	}
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);
		}
	}
Unregister the contao.dns-fallback alias when the model is detached from the registry

@param Registry $registry The model registry