[ermine] Check if story is stopping before adding it.

This change fixes the delay between hitting close button
and story window being removed from the screen.

Change-Id: I40fe80e6af3e6d57586423ec78862ebea1d25c18
diff --git a/shell/ermine/lib/src/models/story_manager.dart b/shell/ermine/lib/src/models/story_manager.dart
index 5494b72..22f0af0 100644
--- a/shell/ermine/lib/src/models/story_manager.dart
+++ b/shell/ermine/lib/src/models/story_manager.dart
@@ -99,7 +99,7 @@
         _storyMap[storyInfo.id].fullscreen =
             storyVisibilityState == StoryVisibilityState.immersive;
       }
-    } else {
+    } else if (storyState != StoryState.stopping) {
       final storyController = StoryControllerProxy();
       await _storyProvider.getController(
         storyInfo.id,