@NonNull
    @Override
    public T getValue() {
        return Objects.requireNonNull(super.getValue());
    }
A data holder class whose content cannot change, however provides notify methods to trigger the registered observers.
Especially useful if another data holder class is used as content.

@param <T> The type of data held by this instance
main/src/main/java/cgeo/geocaching/utils/livedata/ConstantLiveData.java