@Override
    public <K1, V1> KGroupedTable<K1, V1> groupBy(final KeyValueMapper<? super K, ? super V, KeyValue<K1, V1>> selector) {
        return groupBy(selector, Grouped.with(null, null));
    }
@Test
    public void shouldNotAllowNullSelectorOnGroupBy() {
        assertThrows(NullPointerException.class, () -> table.groupBy(null));
    }