src/main/java/io/reactivex/internal/observers/BlockingBaseObserver.java
public final T blockingGet() {
        if (getCount() != 0) {
            try {
                BlockingHelper.verifyNonBlocking();
                await();
            } catch (InterruptedException ex) {
                dispose();
                throw ExceptionHelper.wrapOrThrow(ex);
            }
        }

        Throwable e = error;
        if (e != null) {
            throw ExceptionHelper.wrapOrThrow(e);
        }
        return value;
    }
public final T blockingGet() {
        if (getCount() != 0) {
            try {
                BlockingHelper.verifyNonBlocking();
                await();
            } catch (InterruptedException ex) {
                dispose();
                throw ExceptionHelper.wrapOrThrow(ex);
            }
        }

        Throwable e = error;
        if (e != null) {
            throw ExceptionHelper.wrapOrThrow(e);
        }
        return value;
    }
Block until the first value arrives and return it, otherwise
return null for an empty source and rethrow any exception.
@return the first value or null if the source is empty