defaultLifecycleObserver top-level property
An observer can only be used by one Navigator (include MaterialApp). If you have your own Navigator, please use a new instance of LifecycleObserver.
一个observer只能被一个Navigator（包括MaterialApp）使用。 如果你使用了多个Navigator，请使用新的LifecycleObserver实例。
Implementation
final LifecycleObserver defaultLifecycleObserver = LifecycleObserver();