Add additional flag and update comment

flutter/lib/src/integrations/native_app_start_integration.dart

@@ -154,9 +154,13 @@ class NativeAppStartIntegration extends Integration<SentryFlutterOptions> {
       // is not created therefore we need to create a transaction ourselves.
       // We detect this by checking if the currentRouteName is null.
       // This is a workaround since there is no api that tells us if
-      // the navigator observer exists or not. The currentRouteName is always
-      // set during a didPush triggered by the navigator observer.
-      if (SentryNavigatorObserver.currentRouteName == null) {
+      // the navigator observer exists and has been attached.
+      // The navigator observer also triggers much earlier so if it was attached
+      // it would have already set the routeName and the isCreated flag.
+      // The currentRouteName is always set during a didPush triggered
+      // by the navigator observer.
+      if (!SentryNavigatorObserver.isCreated &&
+          SentryNavigatorObserver.currentRouteName == null) {
         const screenName = SentryNavigatorObserver.rootScreenName;
         final transaction = hub.startTransaction(
             screenName, SentrySpanOperations.uiLoad,

flutter/lib/src/navigation/sentry_navigator_observer.dart

@@ -86,6 +86,7 @@ class SentryNavigatorObserver extends RouteObserver<PageRoute<dynamic>> {
         _routeNameExtractor = routeNameExtractor,
         _additionalInfoProvider = additionalInfoProvider,
         _native = SentryFlutter.native {
+    _isCreated = true;
     if (enableAutoTransactions) {
       _hub.options.sdk.addIntegration('UINavigationTracing');
     }
@@ -121,6 +122,11 @@ class SentryNavigatorObserver extends RouteObserver<PageRoute<dynamic>> {
 
   static String? _currentRouteName;
 
+  static bool _isCreated = false;
+
+  @internal
+  static bool get isCreated => _isCreated;
+
   @internal
   static String? get currentRouteName => _currentRouteName;