match method Null safety
- EngineNotification notification,
 - SpeechCondition condition
 
override
    Match a notification unit
Implementation
@override
Future<bool> match(
    EngineNotification notification, SpeechCondition condition) async {
  if (!await super.match(notification, condition)) return false;
  final metricSettings = EngineSettingsController.instance.item.isMetric;
  final metricNotification = unitIsMetric(notification.parameter);
  final metricCondition =
      condition.parameter?.unit == null || unitIsMetric(condition.parameter);
  return [metricNotification, metricCondition]
      .every((unit) => unit == metricSettings);
}