public void Array_Limit_Null()
        {
            for (int i = 1; i < 10; i++)
            {
                var to = new ISingleSource <int>[]
                {
                    SingleSource.Just(1),
                    null
                }
                .ConcatEagerAll(maxConcurrency: i)
                .Test()
                .WithTag($"maxConcurrency={i}");

                to
                .AssertNotCompleted()
                .AssertError(typeof(NullReferenceException));
            }
        }