Are you using Android for development ?? because if so then maybe you have to change the link from ( localhost:7474/graphql ) into ( ip-address:7474/graphql ). in the ip-address put your computer ip address.. I have faced the same issue and this has solved it for me. For android I use ip-address and for IOS i use localhost.
You can also do this in flutter.
import 'dart:io';
final GraphQLClient client = getClient(
uri: Platform.isAndroid ? 'http://ip-address':7474/graphql : 'http://localhost:7474/graphql',
);