Flutter/Dart get_It singleton locator method failure

I have a simple dart class as follows:

import 'package:flutter/material.dart';

class UiUtils {
  // TEMPORARY FOR UNIT TEST PURPOSES ONLY
  int addition(int x, int y) {
    return x + y;
  }
}

(Note: The above is a sample, the actual class does have more than that temp function.)

My pubspec.yml file contains the following:

dependencies:
  flutter:
    sdk: flutter
  flutter_localizations:
    sdk: flutter
  intl: ^0.17.0

  # The following adds the Cupertino Icons font to your application.
  # Use with the CupertinoIcons class for iOS style icons.
  cupertino_icons: ^1.0.2
  get_it: ^7.1.3
  provider: ^6.0.1
  mockito: ^5.0.16

I have a dependency locator file as such:

import 'package:get_it/get_it.dart';
import 'package:quiz_test/utils/UiUtils.dart';

GetIt dependencyLocator = GetIt.instance;

void setupDependencyLocator() {
  //dependencyLocator.registerSingleton(() => UiUtils());
  dependencyLocator.registerFactory(() => UiUtils());
}

Finally, in main.dart I have the following:

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

(There is of course more code than this).

As it is displayed, the code works fine, however if I change the dependancy_locator file from the current factory method to the singleton instead (i.e. comment out one to enable the other) I get the following error:

[VERBOSE-2:ui_dart_state.cc(209)] Unhandled Exception: type '_ServiceFactory<() => UiUtils, void, void>' is not a subtype of type '_ServiceFactory<Object, dynamic, dynamic>' of 'value'
#0      _LinkedHashMapMixin.[]= (dart:collection-patch/compact_hash.dart)
#1      _GetItImplementation._register (package:get_it/get_it_impl.dart:844:35)
#2      _GetItImplementation.registerSingleton (package:get_it/get_it_impl.dart:587:5)
#3      setupDependencyLocator (package:quiz_test/utils/dependency_locator.dart:7:21)
#4      main (package:quiz_test/main.dart:13:3)
#5      _runMainZoned.<anonymous closure>.<anonymous closure> (dart:ui/hooks.dart:145:25)
#6      _rootRun (dart:async/zone.dart:1428:13)
#7      _CustomZone.run (dart:async/zone.dart:1328:19)
#8      _runZoned (dart:async/zone.dart:1863:10)
#9      runZonedGuarded (dart:async/zone.dart:1851:12)
#10     _runMainZoned.<anonymous closure> (dart:ui/hooks.dart:141:5)
#11     _delayEntrypointInvocation.<anonymous closure> (dart:isolate-patch/isolate_patch.<…>

Can anyone please help me to understand why I cannot use the singleton call rather than the factory one? My thought process is that I do not need a unique instance of this class, which is what I believe factory will give me, I just need a single instance of it for any classes that require it.

Any help is greatly appreciated.

I was able to resolve this by doing the following:

dependencyLocator.registerSingleton<UiUtils>(UiUtils());

So, my dependencyLocator class now looks like this:

import 'package:get_it/get_it.dart';
import 'package:quiz_test/utils/UiUtils.dart';

GetIt dependencyLocator = GetIt.instance;

void setupDependencyLocator() {
  dependencyLocator.registerSingleton<UiUtils>(UiUtils());
}

I hope this helps someone else from getting stuck!