2021-07-30 16:29:06,372 WARN  [org.acm.sch.mes.TimeTableMessagingHandler] (ForkJoinPool.commonPool-worker-5) Error during processing a solver request (10).: java.lang.IllegalArgumentException: The planningId (null) of the member (org.acme.schooltimetabling.domain.Lesson$OptaPlanner$MemberAccessor$Field$id@7e97a96d) of the class (class org.acme.schooltimetabling.domain.Lesson) on externalObject (Math(null)) must not be null.
Maybe initialize the planningId of the class (Lesson) instance (Math(null)) before solving.
Maybe remove the @PlanningId annotation or change the @PlanningSolution annotation's LookUpStrategyType.
        at org.optaplanner.core.impl.domain.lookup.PlanningIdLookUpStrategy.extractPlanningId(PlanningIdLookUpStrategy.java:98)
        at org.optaplanner.core.impl.domain.lookup.PlanningIdLookUpStrategy.addWorkingObject(PlanningIdLookUpStrategy.java:38)
        at org.optaplanner.core.impl.domain.lookup.LookUpManager.addWorkingObject(LookUpManager.java:49)
        at org.optaplanner.core.impl.domain.lookup.LookUpManager.resetWorkingObjects(LookUpManager.java:43)
        at org.optaplanner.core.impl.score.director.AbstractScoreDirector.setWorkingSolution(AbstractScoreDirector.java:181)
        at org.optaplanner.core.impl.score.director.stream.DroolsConstraintStreamScoreDirector.setWorkingSolution(DroolsConstraintStreamScoreDirector.java:66)
        at org.optaplanner.core.impl.solver.scope.SolverScope.setWorkingSolutionFromBestSolution(SolverScope.java:215)
        at org.optaplanner.core.impl.solver.AbstractSolver.solvingStarted(AbstractSolver.java:80)
        at org.optaplanner.core.impl.solver.DefaultSolver.solvingStarted(DefaultSolver.java:200)
        at org.optaplanner.core.impl.solver.DefaultSolver.solve(DefaultSolver.java:174)
        at org.acme.schooltimetabling.messaging.TimeTableMessagingHandler.lambda$solve$0(TimeTableMessagingHandler.java:79)
        at java.base/java.util.concurrent.CompletableFuture$AsyncRun.run(CompletableFuture.java:1736)
        at java.base/java.util.concurrent.CompletableFuture$AsyncRun.exec(CompletableFuture.java:1728)
        at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:290)
        at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1020)
        at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1656)
        at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1594)
        at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:183)