Exception on entity fetch by key

[letarak]

The exception was thrown at the production but could not be reproduced

System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
   at System.Collections.Generic.HashSet`1.Enumerator.MoveNext()
   at Xtensive.Orm.Internals.Prefetch.Fetcher.UpdateCacheFromAllEntityGroupTasks()
   at Xtensive.Orm.Internals.Prefetch.Fetcher.ExecuteTasks(IReadOnlyCollection`1 containers, Boolean skipPersist, Boolean isAsync, CancellationToken token)
   at Xtensive.Orm.Internals.Prefetch.PrefetchManager.ExecuteTasks(Boolean skipPersist, Boolean isAsync, CancellationToken token)
   at Xtensive.Orm.Providers.SqlSessionHandler.FetchEntityState(Key key)
   at Xtensive.Orm.QueryEndpoint.SingleOrDefault(Key key)
   at Xtensive.Orm.QueryEndpoint.SingleOrDefault[T](Key key)