src/main/java/io/reactivex/subjects/BehaviorSubject.java
@Nullable
    public T getValue() {
        Object o = value.get();
        if (NotificationLite.isComplete(o) || NotificationLite.isError(o)) {
            return null;
        }
        return NotificationLite.getValue(o);
    }
@Nullable
    public T getValue() {
        Object o = value.get();
        if (NotificationLite.isComplete(o) || NotificationLite.isError(o)) {
            return null;
        }
        return NotificationLite.getValue(o);
    }
Returns a single value the Subject currently has or null if no such value exists.
<p>The method is thread-safe.
@return a single value the Subject currently has or null if no such value exists