explain Score fails unexpectedly

[JRII972]

I manage to find a solution without a null value and without failure, but when I try to obtain the details of the score (to have the constraints not respected) it fails on a null value.

score_manager.explainScore(_unwrap_java_object(solution.getFinalBestSolution()))

org.optaplanner.jpyinterpreter.types.errors.AttributeErrorTraceback (most recent call last)
org.optaplanner.jpyinterpreter.types.errors.AttributeError: org.optaplanner.jpyinterpreter.types.errors.AttributeError: 'Timeslot' object has no attribute 'id'.

The above exception was the direct cause of the following exception:

java.lang.IllegalStateException           Traceback (most recent call last)
File DefaultScoreManager.java:40, in org.optaplanner.core.impl.score.DefaultScoreManager.explainScore()

java.lang.IllegalStateException: java.lang.IllegalStateException: Joiner (LESS_THAN) threw an exception matching left (Timeslot(id=1672758000.0, datetime=2023-01-03 16:00:00)) and right (None) objects.

The above exception was the direct cause of the following exception:

While solution.getFinalBestSolution() works without problem

@optapy.planning_entity
class Entity:
    def __init__(self, code, actual_value):
        self.code = code
        self.date = self.get_possible_date()[0]
        self.date_end = Timeslot( self.date._datetime + timedelta(hours=1))

   ...

    @optapy.planning_variable([Timeslot], value_range_provider_refs=['date_range'])
    def get_date(self): return self.date

    def set_date(self, date):  self.date = date

    def get_date_end(self):
        return Timeslot( self.date._datetime + timedelta(hours=1))

    def set_date_end(self, date_end): pass

   ...

@problem_fact
class Timeslot:
    _datetime:datetime
    def __init__(self, _datetime:datetime):
        self._datetime = _datetime

    @property
    def id(self) : return self._datetime.timestamp()

    ...

The only constraint :

constraint_factory.for_each_unique_pair(Entity,
                                                constraint.Joiners.overlapping(lambda entity: entity.date, lambda entity: entity.date_end))
        .penalize('Same value', optapy.score.SimpleScore.ONE)