Exception in thread "AWT-EventQueue-0" java.lang.IllegalStateException: Solving failed.
	at org.optaplanner.examples.common.swingui.SolverAndPersistenceFrame$SolveWorker.done(SolverAndPersistenceFrame.java:377)
	at java.desktop/javax.swing.SwingWorker$5.run(SwingWorker.java:750)
	at java.desktop/javax.swing.SwingWorker$DoSubmitAccumulativeRunnable.run(SwingWorker.java:848)
	at java.desktop/sun.swing.AccumulativeRunnable.run(AccumulativeRunnable.java:112)
	at java.desktop/javax.swing.SwingWorker$DoSubmitAccumulativeRunnable.actionPerformed(SwingWorker.java:858)
	at java.desktop/javax.swing.Timer.fireActionPerformed(Timer.java:311)
	at java.desktop/javax.swing.Timer$DoPostEvent.run(Timer.java:243)
	at java.desktop/java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:318)
	at java.desktop/java.awt.EventQueue.dispatchEventImpl(EventQueue.java:771)
	at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:722)
	at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:716)
	at java.base/java.security.AccessController.doPrivileged(AccessController.java:399)
	at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:86)
	at java.desktop/java.awt.EventQueue.dispatchEvent(EventQueue.java:741)
	at java.desktop/java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:203)
	at java.desktop/java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:124)
	at java.desktop/java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:113)
	at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:109)
	at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
	at java.desktop/java.awt.EventDispatchThread.run(EventDispatchThread.java:90)
Caused by: java.lang.IllegalStateException: Solver threw an exception.
	at org.optaplanner.examples.common.business.SolutionBusiness.solve(SolutionBusiness.java:364)
	at org.optaplanner.examples.common.swingui.SolverAndPersistenceFrame$SolveWorker.doInBackground(SolverAndPersistenceFrame.java:365)
	at java.desktop/javax.swing.SwingWorker$1.call(SwingWorker.java:304)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at java.desktop/javax.swing.SwingWorker.run(SwingWorker.java:343)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
	at java.base/java.lang.Thread.run(Thread.java:833)
Caused by: java.util.concurrent.ExecutionException: java.lang.IllegalStateException: Solving failed for problemId (1).
	at java.base/java.util.concurrent.FutureTask.report(FutureTask.java:122)
	at java.base/java.util.concurrent.FutureTask.get(FutureTask.java:191)
	at org.optaplanner.core.impl.solver.DefaultSolverJob.getFinalBestSolution(DefaultSolverJob.java:215)
	at org.optaplanner.examples.common.business.SolutionBusiness.solve(SolutionBusiness.java:359)
	... 7 more
Caused by: java.lang.IllegalStateException: Solving failed for problemId (1).
	at org.optaplanner.core.impl.solver.DefaultSolverJob.call(DefaultSolverJob.java:131)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	... 3 more
Caused by: java.lang.IllegalStateException: The supply (ExternalizedSingletonInverseVariableSupply(previousStandstill)) is corrupted, because the entity (Bern) for sourceVariable (previousStandstill) cannot be retracted: the entity was never inserted for that value (Bern).
	at org.optaplanner.core.impl.domain.variable.inverserelation.ExternalizedSingletonInverseVariableSupply.retract(ExternalizedSingletonInverseVariableSupply.java:111)
	at org.optaplanner.core.impl.domain.variable.inverserelation.ExternalizedSingletonInverseVariableSupply.beforeVariableChanged(ExternalizedSingletonInverseVariableSupply.java:69)
	at org.optaplanner.core.impl.domain.variable.listener.support.VariableChangedNotification.triggerBefore(VariableChangedNotification.java:31)
	at org.optaplanner.core.impl.domain.variable.listener.support.VariableListenerNotifiable.addNotification(VariableListenerNotifiable.java:57)
	at org.optaplanner.core.impl.domain.variable.listener.support.VariableListenerSupport.beforeVariableChanged(VariableListenerSupport.java:144)
	at org.optaplanner.core.impl.score.director.AbstractScoreDirector.beforeVariableChanged(AbstractScoreDirector.java:401)
	at org.optaplanner.core.impl.score.director.AbstractScoreDirector.changeVariableFacade(AbstractScoreDirector.java:416)
	at org.optaplanner.core.impl.heuristic.selector.move.generic.chained.ChainedChangeMove.doMoveOnGenuineVariables(ChainedChangeMove.java:76)
	at org.optaplanner.core.impl.heuristic.move.AbstractMove.doMoveOnly(AbstractMove.java:42)
	at org.optaplanner.core.impl.heuristic.move.AbstractMove.doMove(AbstractMove.java:36)
	at org.optaplanner.core.impl.heuristic.move.AbstractMove.doMove(AbstractMove.java:31)
	at org.optaplanner.core.impl.score.director.AbstractScoreDirector.doAndProcessMove(AbstractScoreDirector.java:234)
	at org.optaplanner.core.impl.localsearch.decider.LocalSearchDecider.doMove(LocalSearchDecider.java:133)
	at org.optaplanner.core.impl.localsearch.decider.LocalSearchDecider.decideNextStep(LocalSearchDecider.java:117)
	at org.optaplanner.core.impl.localsearch.DefaultLocalSearchPhase.solve(DefaultLocalSearchPhase.java:88)
	at org.optaplanner.core.impl.solver.AbstractSolver.runPhases(AbstractSolver.java:99)
	at org.optaplanner.core.impl.solver.DefaultSolver.solve(DefaultSolver.java:209)
	at org.optaplanner.core.impl.solver.DefaultSolverJob.call(DefaultSolverJob.java:125)
	... 4 more