public final void setSchedulerSupplier(Supplier<Scheduler> schedulerSupplier) {
        this.schedulerSupplier = checkNotNull(schedulerSupplier);
    }
@Test
    public void test_setSchedulerSupplier_whenNull() {
        ReactorBuilder builder = newBuilder();
        assertThrows(NullPointerException.class, () -> builder.setSchedulerSupplier(null));
    }