neon: Fix browser not opening on login page

packages/neon/lib/src/pages/login.dart

@@ -32,16 +32,16 @@ class _LoginPageState extends State<LoginPage> {
       _loginBloc.setServerURL(widget.serverURL);
     }
 
-    _loginBloc.loginFlowInit.listen((final init) {
+    WidgetsBinding.instance.addPostFrameCallback((final _) {
+      _loginBloc.loginFlowInit.listen((final init) async {
         if (init != null && !Provider.of<NeonPlatform>(context, listen: false).canUseWebView) {
-        WidgetsBinding.instance.addPostFrameCallback((final _) async {
           await launchUrlString(
             init.login,
             mode: LaunchMode.externalApplication,
           );
-        });
         }
       });
+    });
 
     _loginBloc.loginFlowResult.listen((final result) async {
       if (result != null) {