public static function & GetById ($formId) {
		if (isset(self::$instances[$formId])) {
			return self::$instances[$formId];
		} else {
			throw new \RuntimeException(
				'No form instance exists under form id `'.$formId.'`.'
				. ' Check if searched form instance has been already initialized'
				. ' or if form id has been already set.'
			);
		}
	}