In case someone came here thanks to google, searching about how to return null if firstWhere found nothing, when your app is Null Safe, use the new method of package:collection called firstWhereOrNull.
import 'package:collection/collection.dart'; // You have to add this manually, for some reason it cannot be added automatically
// somewhere...
MyStuff? stuff = someStuffs.firstWhereOrNull((element) => element.id == 'Cat');
About the method:
https://pub.dev/documentation/collection/latest/collection/IterableExtension/firstWhereOrNull.html