lib/sign_in_screen.dart

// ...

class _SignInScreenState extends State<SignInScreen> {
  // ...
  
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Form(
        key: _formKey,
        child: Center(
          child: Padding(
            padding: const EdgeInsets.symmetric(horizontal: 24),
            child: Column(
              mainAxisAlignment: MainAxisAlignment.center,
              children: [
                // ...
                SizedBox(
                  width: double.infinity,
                  child: ElevatedButton(
                    onPressed: () => _onSignIn(),
                    child: Text('ログイン'),
                  ),
                ),
                // ...
              ],
            ),
          ),
        ),
      ),
    );
  }

  Future<void> _onSignIn() async {
    try {
      if (_formKey.currentState?.validate() != true) {
        return;
      }

      // 新規登録と同じく入力された内容をもとにログイン処理を行う
      final String email = _emailController.text;
      final String password = _passwordController.text;
      await FirebaseAuth.instance
          .signInWithEmailAndPassword(email: email, password: password);

      Navigator.of(context).pushReplacement(
        MaterialPageRoute(
          builder: (_) => PhotoListScreen(),
        ),
      );
    } catch (e) {
      // 失敗したらエラーメッセージを表示
      await showDialog(
        context: context,
        builder: (context) {
          return AlertDialog(
            title: Text('エラー'),
            content: Text(e.toString()),
          );
        },
      );
    }
  }

  // ...
}