@Override
    public KTable<Windowed<K>, V> aggregate(final Initializer<V> initializer,
                                            final Merger<? super K, V> sessionMerger) {
        return aggregate(initializer, sessionMerger, Materialized.with(null, null));
    }
@Test
    public void shouldNotHaveNullInitializer2OnAggregate() {
        assertThrows(NullPointerException.class, () -> windowedCogroupedStream.aggregate(null, sessionMerger,
            Materialized.as("test")));
    }