0.4.8 - Parallel queries may produce java.util.ConcurrentModificationException

[martinreilent]

Got this exception today after experimenting with 0.4.8.

Setup:
Spring boot fresh setup 2.2.7
Oracle db 12c

concurrentModification_stackTrace.txt

Tried to also reproduce with h2 in memory test data

@EntityScan("com.example.graghql.graphql_learning.models")
public class GraphqlLearningApplication {

    public static void main(String[] args) {
        SpringApplication.run(GraphqlLearningApplication.class, args);
    }

    @Bean
    CommandLineRunner asd (GraphQLExecutor executor) {
        return args -> {
            String query = "query { Authors(where: { books: {genre: {IN: %s}} }) { select { id name books { id title genre } } } }";
            ExecutorService executorService = Executors.newFixedThreadPool(10);

            for (Genre genre: Genre.values()) {
                executorService.submit(() ->  executor.execute(String.format(query, genre.toString()), Map.of()).getData());
            }
        };
    }

but couldnt get the same exact result. Instead got the
java.util.concurrent.CompletionException: java.lang.NullPointerException

parallel_execute_npe.txt

[hump]

we also got such errors with the actual version while use using a react backend...

[hump]

happens since 0.4.0

[hump]

any chance this could be fixed?
would a test case helpful to reproduce it?