[iOS] Unhandled Exception: Null check operator used on a null value

iOS, Native to Flutter success,
Flutter pop to Native, failed.

log:

```
[VERBOSE-2:dart_vm_initializer.cc(41)] Unhandled Exception: Null check operator used on a null value
#0 FlutterBoostAppState.pop (package:flutter_boost/src/flutter_boost_app.dart:497:51)
#1 FlutterBoostAppState.popWithResult (package:flutter_boost/src/flutter_boost_app.dart:391:18)
#2 BoostNavigator.pop (package:flutter_boost/src/boost_navigator.dart:109:28)
#3 NavigatorExtState.pop (package:flutter_boost/src/boost_container.dart:222:31)
#4 _SplashPageState.build. (package:ost_flutter_module/splash/view.dart:55:39)
#5 _InkResponseState.handleTap (package:flutter/src/material/ink_well.dart:1154:21)
#6 GestureRecognizer.invokeCallback (package:flutter/src/gestures/recognizer.dart:275:24)
#7 TapGestureRecognizer.handleTapUp (package:flutter/src/gestures/tap.dart:654:11)
#8 BaseTapGestureRecognizer._checkUp (package:flutter/src/gestures/tap.dart:311:5)
#9 BaseTapGestureRecognizer.handlePrimaryPointer (package:flutter/src/gestures/tap.dart:244:7)
#10 PrimaryPointerGestureRecognizer.handleEvent (package:flutter/src/gestures/recognizer.dart:630:9)
#11 PointerRouter._dispatch (package:flutter/src/gestures/pointer_router.dart:98:12)
#12 PointerRouter._dispatchEventToRoutes. (package:flutter/src/gestures/pointer_router.dart:143:9)
#13 _LinkedHashMapMixin.forEach (dart:collection-patch/compact_hash.dart:625:13)
#14 PointerRouter._dispatchEventToRoutes (package:flutter/src/gestures/pointer_router.dart:141:18)
#15 PointerRouter.route (package:flutter/src/gestures/pointer_router.dart:127:7)
#16 GestureBinding.handleEvent (package:flutter/src/gestures/binding.dart:465:19)
#17 GestureBinding.dispatchEvent (package:flutter/src/gestures/binding.dart:445:22)
#18 RendererBinding.dispatchEvent (package:flutter/src/rendering/binding.dart:331:11)
#19 GestureBinding._handlePointerEventImmediately (package:flutter/src/gestures/binding.dart:400:7)
#20 GestureBinding.handlePointerEvent (package:flutter/src/gestures/binding.dart:363:5)
#21 GestureBinding._flushPointerEventQueue (package:flutter/src/gestures/binding.dart:320:7)
#22 GestureBinding._handlePointerDataPacket (package:flutter/src/gestures/binding.dart:293:9)
#23 _invoke1 (dart:ui/hooks.dart:158:13)
#24 PlatformDispatcher._dispatchPointerDataPacket (dart:ui/platform_dispatcher.dart:382:7)
#25 _dispatchPointerDataPacket (dart:ui/hooks.dart:91:31)
```

感觉是boostNavigator appState 为null导致的。

Start in lib/flutter_boost_app.dart at pop (line 497), then trace the popWithResult and BoostNavigator.pop calls shown in the stack trace. Reproduce the iOS native-to-Flutter and Flutter-to-native navigation flow and determine why the app state is null. Done means the Flutter-to-native pop completes without an unhandled null-check exception.

Written by the indexing model from the issue text.