migrate sample app

packages/stream_chat_v1/lib/stream_version.dart

@@ -5,7 +5,7 @@ import 'package:stream_chat_flutter/stream_chat_flutter.dart';
 
 class StreamVersion extends StatelessWidget {
   const StreamVersion({
-    Key key,
+    Key? key,
   }) : super(key: key);
 
   @override
@@ -20,7 +20,7 @@ class StreamVersion extends StatelessWidget {
             return SizedBox();
           }
 
-          final pubspec = snapshot.data;
+          final pubspec = snapshot.data!;
           final yaml = loadYaml(pubspec);
           final streamChatDep =
               yaml['packages']['stream_chat_flutter']['version'];