@Override
    public <VR> KTable<Windowed<K>, VR> aggregate(final Initializer<VR> initializer,
                                                  final Aggregator<? super K, ? super V, VR> aggregator) {
        return aggregate(initializer, aggregator, Materialized.with(keySerde, null));
    }
@Test
    public void shouldThrowNullPointerOnAggregateIfAggregatorIsNull() {
        assertThrows(NullPointerException.class, () -> windowedStream.aggregate(MockInitializer.STRING_INIT, null));
    }