Imagine the following constraint:

constraintFactory.from(Cluster.class)
                .ifExists(Pod.class, 
                   Joiners.equal(Function.identity(), pod -> pod.getNode().getCluster()))

The constraint throws a NPE as pod.getNode() is null, because pod is the PlanningEntity and its reference to Node is the PlanningVariable.

In this context, the ifExists() behaves like fromUnfiltered(), without any option to avoid the uninitialized planning entities.