This code sample uses Records or Custom Result class to pass data and exceptions to the UI.

core.dart

import 'package:example_user_app/result.dart';
import 'package:example_user_app/user.dart';

abstract class Core {
  ({User? user, Exception? ex}) getUser();
  Result getFilterNames({required String name});
}

class CoreImpl implements Core {
  User? user;

  ///using records approach
  @override
  ({User? user, Exception? ex}) getUser() {
    user = User(firstName: 'Kofi', lastName: 'Mensah',age: 25); ///Add age: 25 to bypass error

//no age added so throw error
    if (user?.age == null) {
      final ex = Exception('User age not present');
      return (user: null, ex: ex);
    }
    user?.age = 22;
    return (user: user, ex: null);
  }

  ///using custom class approach
  @override
  Result getFilterNames({required String name}) {
    Result res = Result();
    final users = ['Ama', 'Kofi'];

    final results = users
        .where((names) => names.toLowerCase().contains(name.toLowerCase()))
        .toList();
    if (results.isNotEmpty) {
      res.result = results;
    } else if (results.isEmpty) {
      res.exception = Exception('No user found');
    } else {
      res.exception = Exception('Something went wrong!');
    }
    return res;
  }
}

main.dart

import 'package:example_user_app/core.dart';
import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(primarySwatch: Colors.purple, useMaterial3: true),
      home: const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({super.key, required this.title});

  final String title;

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  final core = CoreImpl();
  String results = '';

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Padding(
          padding: const EdgeInsets.all(16.0),
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              const Spacer(),
              TextFormField(
                decoration:
                    const InputDecoration(hintText: 'Search for name here'),
                onChanged: (value) {
                  if (value.isNotEmpty && value.length >= 3) {
                    searchNames(value);
                  } else {
                    setState(() {
                      results = '';
                    });
                  }
                },
              ),
              const SizedBox(height: 10),
              Text(
                results,
                style: Theme.of(context).textTheme.headlineMedium,
              ),
              const Spacer(),
            ],
          ),
        ),
      ),
      floatingActionButton: FloatingActionButton.extended(
        onPressed: () {
          final res = core.getUser();
          if (res.ex != null) {
            ScaffoldMessenger.of(context)
                .showSnackBar(SnackBar(content: Text(res.ex!.toString())));
          }
          if (res.user != null) {
            setState(() {
              results = res.user!.toString();
            });
          }
        },
        tooltip: 'Get user',
        label: const Text('Get user'),
        icon: const Icon(CupertinoIcons.person_2_alt),
      ),
    );
  }

  void searchNames(String name) {
    final res = core.getFilterNames(name: name);
    if (res.exception != null) {
      ScaffoldMessenger.of(context)
          .showSnackBar(SnackBar(content: Text(res.exception!.toString())));
    }
    if (res.result != null) {
      final names = res.result as List<String>;
      setState(() {
        results = names.join(',\n');
      });
    }
  }
}

result.dart

///custom result class
class Result<T> {
  T? result;
  Exception? exception;

  Result({this.result, this.exception});
}

user.dart

///user model
class User {
  String? firstName;
  String? lastName;
  int? age;

  User({this.firstName, this.lastName, this.age});

  User.fromJson(Map<String, dynamic> json) {
    firstName = json['first_name'];
    lastName = json['last_name'];
    age = json['age'];
  }

  Map<String, dynamic> toJson() {
    final Map<String, dynamic> data = {};
    data['first_name'] = firstName;
    data['last_name'] = lastName;
    data['age'] = age;
    return data;
  }
}