2.x: WithLatestFrom not calling onSubscribe() on `actual` observer

[ZacSweers]

We're seeing an NPE from the s field of SerializedObserver not being set when used with .withLatestFrom(). Digging around in the source, it looks like it internally is not calling onSubscribe() to the actual observer field.

Example trace looks like this:

java.lang.NullPointerException
	at io.reactivex.observers.SerializedObserver.onNext(SerializedObserver.java:91)
	at io.reactivex.internal.operators.observable.ObservableWithLatestFrom$WithLatestFromObserver.onNext(ObservableWithLatestFrom.java:101)
	at io.reactivex.internal.operators.observable.ObservableFilter$FilterObserver.onNext(ObservableFilter.java:51)
	at io.reactivex.internal.operators.observable.ObservableMap$MapObserver.onNext(ObservableMap.java:63)
	at io.reactivex.subjects.PublishSubject$PublishDisposable.onNext(PublishSubject.java:263)
	at io.reactivex.subjects.PublishSubject.onNext(PublishSubject.java:182)

The snippet in SerializedObserver that corresponds to

    @Override
    public void onNext(T t) {
        if (done) {
            return;
        }
        if (t == null) {
            s.dispose();  // <----- this line, "s" is null because onSubscribe was never called
            onError(new NullPointerException("onNext called with null. Null values are generally not allowed in 2.x operators and sources."));
            return;
        }

[ZacSweers]

Specifically, the static WithLatestFromObserver class in ObservableWithLatestFrom never calls OnSubscribe on its actual before sending other events to it.

[akarnokd]

The main problem is that withLatestFrom doesn't check for null returned by the combiner. I'll post a PR shortly.

[ZacSweers]

👍 thanks!

[ZacSweers]

Fixed in #4970