Revert "compilation error"

This reverts commit 3ac1608.

Author: nightwolf-chen

lib/container/container_manager.dart

@@ -28,7 +28,7 @@ import 'package:flutter_boost/container/boost_container.dart';
 import 'package:flutter_boost/container/container_coordinator.dart';
 import 'package:flutter_boost/flutter_boost.dart';
 import 'package:flutter_boost/support/logger.dart';
-
+import 'package:flutter_boost/container/navigator_observer.dart';
 enum ContainerOperation { Push, Onstage, Pop, Remove }
 
 typedef BoostContainerObserver = void Function(
@@ -100,6 +100,8 @@ class ContainerManagerState extends State<BoostContainerManager> {
     assert(widget.initNavigator != null);
     _onstage = BoostContainer.copy(widget.initNavigator);
 
+    FlutterBoost.containerManager.navigatorObserver.addBoostNavigatorObserver(NavigatorCanPopObserver());
+
     WidgetsBinding.instance.addPostFrameCallback((_) {
       setState(() {});
     });