Fix a few notices

lib/forez/tour/join_group_pane.dart

@@ -36,8 +36,7 @@ class _JoinGroupPaneBody extends StatefulWidget {
   const _JoinGroupPaneBody({
     Key? key,
     required this.onUpdated,
-  })  : assert(onUpdated != null),
-        super(key: key);
+  })  : super(key: key);
 
   @override
   JoinGroupPaneBodyState createState() => JoinGroupPaneBodyState();