How to pass parameters to the sub page through onGenerateRoute in flutter program

I’m following a tutorial on a website to study flutter/dart. the tutorial techs me how to pass parameters (a Map) in named route through onGenerateRoute. now, I have created 2 webpage, say, main page and sub page, on my main page I created a button, if I clicked the button, it would route me to sub page with parameters. however, my program always stuck at error
======== Exception caught by widgets library =======================================================
The following _TypeError was thrown building Builder(dirty):
type ‘Null’ is not a subtype of type ‘Map<dynamic, dynamic>’

It’s frustrated after many attempts. I appreciate that if somebody can show root cause in detailed.
I’m using flutter 3.16.5, and dart is 3.2.3

I checked the variables

by print(settings); 
print(settings.arguments);           
print(settings.name);
it returns 
I/flutter ( 4675): RouteSettings("/subpage", {title: pass parameter in named route})
I/flutter ( 4675): {title: pass parameter in named route}
I/flutter ( 4675): /subpage

I don’t understand why the flutter still reported type error

below program for your reference:

//-----------------------------this is main page
import 'package:flutter/material.dart';
import 'package:firstapp/testsubpage.dart';

void main(){
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  final Map routes={
    '/': (context) => const ShowPage(),
    '/subpage':(context,{arguments}) => SubPage(args: arguments,),
  };

  MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        title: 'Flutter Demo',
        debugShowCheckedModeBanner: false,
        theme: ThemeData(
            useMaterial3:true,
            colorScheme: ColorScheme.fromSeed(seedColor: Colors.blue),
            appBarTheme: const AppBarTheme(backgroundColor: Colors.blue)
        ),
//      home: const ShowPage()
        initialRoute: '/',
        onGenerateRoute:(RouteSettings settings){
          String? name=settings.name;
          Function pageContentBuilder = routes[name];
          if(pageContentBuilder != null){
            if(settings.arguments !=null){
              final Route route=MaterialPageRoute(builder: (context) => pageContentBuilder(settings.arguments));

              print(settings);
              print(settings.arguments);
              print(settings.name);

//              return MaterialPageRoute(builder: (context){
//                return pageContentBuilder('/subpage', settings.arguments);});
              return route;
            }else{
              final Route route=MaterialPageRoute(builder: (context)=>pageContentBuilder(context));
              return route;
            }
          }
          return null;
        }
    );
  }
}

class ShowPage extends StatefulWidget {
  const ShowPage({super.key});

  @override
  State<ShowPage> createState() => _ShowPageState();
}

class _ShowPageState extends State<ShowPage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('Flutter Named Route'),),
      body: Center(
          child: OutlinedButton(onPressed: () {
            Navigator.pushNamed(context, '/subpage', arguments: {'title':'pass parameter in named route'});
          }, child: const Text('Click Me'),)
      )
      ,
    );
  }
}


//-----this is sub page
import 'package:flutter/material.dart';

class SubPage extends StatefulWidget {
  final Map args;
  const SubPage({super.key, required this.args});
  @override
  State<SubPage> createState() => _SubPageState();
}

class _SubPageState extends State<SubPage> {

  @override
  void initState() {
    // TODO: implement initState
    super.initState();
  }
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title:  Text(widget.args['title']),),
      body: const Center(
        child: Text('This is sub page'),
      ),
    );
  }
}