AlertDialog is not working on onPressed() – Flutter

trying to do simple login form and want to test the result.

I want to show a message on dialog box but when I am trying to use showDialog and AlertDialog but, it is not working. I have tried debugPrint/Print/log and other functions but nothing worked . it will not print it on console as well.

can anyone please help me?

I want to print message on dialog box.

import 'package:flutter/material.dart';

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

  @override
  State<Login> createState() => _LoginState();
}

class _LoginState extends State<Login> {
  late bool _autovalidate = false;

  // Message Stored
  late String message;

  // login credentials var
  late String userName = 'admin';
  late String passWord = '123';

  // TextEditingController initiated
  final name = TextEditingController();
  final password = TextEditingController();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(
          'Expense Manager',
          style: TextStyle(
            color: Colors.white,
            fontWeight: FontWeight.bold,
          ),
        ),
      ),
      body: SingleChildScrollView(
        child: Form(
          child: Card(
            shape: RoundedRectangleBorder(
              borderRadius: BorderRadius.circular(15.0),
            ),
            child: Column(
              children: <Widget>[
                Padding(
                  padding: EdgeInsets.all(10.0),
                ),
                Image(
                  image: AssetImage('images/logo.png'),
                  height: 100.0,
                  width: 100.0,
                ),
                ListTile(
                  leading: Icon(Icons.person),
                  title: TextFormField(
                    controller: name,
                    validator: (input) {
                      if (input!.isEmpty) {
                        return 'Enter Username';
                      }
                    },
                    decoration: InputDecoration(labelText: 'Username'),
                  ),
                ),
                ListTile(
                  leading: Icon(Icons.password),
                  title: TextFormField(
                    controller: password,
                    obscureText: true,
                    validator: (input) {
                      if (input!.isEmpty) {
                        return 'Enter Password';
                      }
                    },
                    decoration: InputDecoration(labelText: 'Password'),
                  ),
                ),
                Container(
                  padding: EdgeInsets.all(20.0),
                  child: ButtonTheme(
                    height: 40.0,
                    minWidth: 200.0,
                    child: ElevatedButton(
                      onPressed: () {
                        _showAppLogin();
                      },
                      child: Text(
                        'Login',
                        style: TextStyle(
                          color: Colors.white,
                        ),
                      ),
                      style: ElevatedButton.styleFrom(
                        shape: RoundedRectangleBorder(
                          borderRadius: BorderRadius.circular(10.0),
                        ),
                        backgroundColor: Colors.redAccent,
                        padding: EdgeInsets.zero,
                        tapTargetSize: MaterialTapTargetSize.shrinkWrap,
                      ),
                    ),
                  ),
                ),
              ],
            ),
          ),
        ),
      ),
    );
  }

  _showAppLogin() {
    if (name.text == null && password.text == null) {
      if (name.text == userName && password.text == passWord) {
        _showAlertDialog('Login Successful');
      } else {
        setState(() {
          _autovalidate = true;
        });
        _showAlertDialog('Invalid Credentials');
      }
    }
  }

  void _showAlertDialog(String message) {
    AlertDialog alertDialog = AlertDialog(
      icon: Icon(Icons.message_outlined),
      content: Text(message),
    );
    showDialog(context: context, builder: (_) => alertDialog);
  }
}