WidgetTestFutureBuilder.dart

  testWidgets("Show error text widget when future returns error",
      (WidgetTester tester) async {
    Future<List<Contact>>? future;
    final contactsScreen = MaterialApp(
      home: buildContactList(
        contacts: future,
      ),
    );
    await tester.pumpWidget(contactsScreen);
    await tester.pumpAndSettle();
    final errorContactText = find.text("Oops contact load failed !");
    expect(errorContactText, findsOneWidget);
  });

Widget buildContactList({required contacts}) {
  return FutureBuilder<List<Contact>>(
    future: contacts,
    builder: (context, snapshot) {
      if (snapshot.hasData) {
        return ListView.builder(
            itemCount: snapshot.data!.length,
            itemBuilder: (context, index) {
              return Card(
                margin: EdgeInsets.all(10.0),
                child: ListTile(
                  leading: CircleAvatar(
                    radius: 40,
                    backgroundImage: AssetImage("assets/icons/img_avatar.png"),
                  ),
                  title: Text(snapshot.data![index].getName()),
                  subtitle: Text(snapshot.data![index].getContactNumber()),
                ),
              );
            });
      } else if (snapshot.hasError) {
        print("haserror");
        return Text(
          'Oops contact load failed !',
          style: TextStyle(
            color: Colors.red,
            fontSize: 20,
          ),
        );
      } else if (snapshot.data == null) {
        return Text('No Contacts');
      }
      return Center(
        child: const CircularProgressIndicator(),
      );
    },
  );
}