How to redirect if token expired in flutter with http

I have a problem where when the user’s token expires it should return to the login page, but the case I experienced was that it didn’t return to the login page, instead it gave an exception in the form of text ‘Token Expired’ in the widget.

is there something wrong with my code.

this is the code in the login page widget

 Container(
                      width: double.infinity,
                      height: 45,
                      child: isLoading
                          ? Center(
                              child: CircularProgressIndicator(
                                color: primaryColor,
                              ),
                            )
                          : CustomButtonFilled(
                              title: 'Login',
                              onPressed: () async {
                                final prefs =
                                    await SharedPreferences.getInstance();
                                prefs.setString(Constant.token, '');
                                if (nimController.text.isEmpty ||
                                    passwordController.text.isEmpty) {
                                  showError('NIM/Password harus diisi');
                                } else {
                                  setState(() {
                                    isLoading = true;
                                  });
                                  User? user = await userProvider.login(
                                    nimController.text,
                                    passwordController.text,
                                  );
                                  setState(() {
                                    isLoading = false;
                                  });
                                  if (user == null) {
                                    showError('NIM/Password tidak sesuai!');
                                  } else {
                                    userProvider.user = user;
                                    // ignore: use_build_context_synchronously
                                    Navigator.pushNamedAndRemoveUntil(
                                      context,
                                      '/main',
                                      (route) => false,
                                    );
                                  }
                                }
                              },
                            ),
                    ),

call API login

 Future<User?> login(String nim, String password) async {
    String url = Constant.baseURL;        try {
      var body = {
        'username': nim,
        'password': password,
      };
      var response = await http.post(
        Uri.parse(
          '$url/login_mhs',
        ),
        body: body,
      );
      if (response.statusCode == 200) {
        final token = jsonDecode(response.body)['data']['access_token'];
        await UtilSharedPreferences.setToken(token);
        print(token);
        return User.fromJson(jsonDecode(response.body));
      } else {
        return null;
      }
    } catch (e) {
      print(e);
      throw Exception();
    }
  }

widget splashscreen

Future<void> toLogin() async {
    Timer(
      const Duration(seconds: 3),
      () async {
        SharedPreferences prefs = await SharedPreferences.getInstance();
        String? token = prefs.getString(Constant.token);
        // ignore: use_build_context_synchronously
        Navigator.pushReplacementNamed(
          context,
          token != null ? AppRoute.mainRoute : AppRoute.loginRoute,
          arguments: token,
        );
      },
    );
  }

  @override
  void initState() {
    super.initState();
    toLogin();
  }