fix: Downgrade main.dart

lib/main.dart

@@ -1,4 +1,4 @@
-// @dart=2.12
+// @dart=2.11
 
 import 'dart:async';
 
@@ -68,14 +68,14 @@ void main() async {
 }
 
 class FluffyChatApp extends StatefulWidget {
-  final Widget? testWidget;
+  final Widget testWidget;
   final List<Client> clients;
-  final Map<String, String>? queryParameters;
+  final Map<String, String> queryParameters;
 
   const FluffyChatApp({
-    Key? key,
+    Key key,
     this.testWidget,
-    required this.clients,
+    @required this.clients,
     this.queryParameters,
   }) : super(key: key);
 
@@ -89,9 +89,9 @@ class FluffyChatApp extends StatefulWidget {
 }
 
 class _FluffyChatAppState extends State<FluffyChatApp> {
-  GlobalKey<VRouterState>? _router;
-  bool? columnMode;
-  String? _initialUrl;
+  GlobalKey<VRouterState> _router;
+  bool columnMode;
+  String _initialUrl;
 
   @override
   void initState() {
@@ -139,8 +139,8 @@ class _FluffyChatAppState extends State<FluffyChatApp> {
                 : null,
             routes: AppRoutes(columnMode ?? false).routes,
             builder: (context, child) {
-              LoadingDialog.defaultTitle = L10n.of(context)!.loadingPleaseWait;
-              LoadingDialog.defaultBackLabel = L10n.of(context)!.close;
+              LoadingDialog.defaultTitle = L10n.of(context).loadingPleaseWait;
+              LoadingDialog.defaultBackLabel = L10n.of(context).close;
               LoadingDialog.defaultOnError =
                   (e) => (e as Object).toLocalizedString(context);
               WidgetsBinding.instance?.addPostFrameCallback((_) {