public function nullRef(string $type): Reference
	{
		foreach ($this->backends as $b => $backend) {
			$m = $backend->getMachine($this, $type);
			if ($m !== null) {
				$ref = $m->nullRef();

				// Emit events
				if ($this->debug_logger) {
					$this->debug_logger->afterReferenceCreated($backend, $ref);
				}
				if ($this->after_reference_created) {
					$this->after_reference_created->emit($ref);
				}

				return $ref;
			}
		}
		throw new RuntimeException('Cannot create machine: '.$type);
	}