src/main/java/io/reactivex/disposables/SerialDisposable.java
@Nullable
    public Disposable get() {
        Disposable d = resource.get();
        if (d == DisposableHelper.DISPOSED) {
            return Disposables.disposed();
        }
        return d;
    }
@Nullable
    public Disposable get() {
        Disposable d = resource.get();
        if (d == DisposableHelper.DISPOSED) {
            return Disposables.disposed();
        }
        return d;
    }
Returns the currently contained Disposable or null if this container is empty.
@return the current Disposable, may be null