我正在研究我在github上找到的代码，因为我正在做一个类似的项目。这是链接：https://github.com/ge0ffrey/optaplanner-mixedvrp-experiment我尝试运行相同的代码，得到了一个IllegalStateException，跟踪如下：

Exception in thread "AWT-EventQueue-0" java.lang.IllegalStateException: Solving failed.
    at org.optaplanner.examples.common.swingui.SolverAndPersistenceFrame$SolveWorker.done(SolverAndPersistenceFrame.java:381)
    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:773)
    at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:720)
    at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:714)
    at java.base/java.security.AccessController.doPrivileged(AccessController.java:400)
    at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:87)
    at java.desktop/java.awt.EventQueue.dispatchEvent(EventQueue.java:742)
    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: The entity (Visit-18) has a variable (previousStandstill) with value (Visit-25) which has a sourceVariableName variable (nextVisit) with a value (Visit-12) which is not null.
Verify the consistency of your input problem for that sourceVariableName variable.
    at org.optaplanner.core.impl.domain.variable.inverserelation.SingletonInverseVariableListener.insert(SingletonInverseVariableListener.java:74)
    at org.optaplanner.core.impl.domain.variable.inverserelation.SingletonInverseVariableListener.afterVariableChanged(SingletonInverseVariableListener.java:53)
    at org.optaplanner.core.impl.domain.variable.listener.support.VariableListenerSupport.triggerVariableListenersInNotificationQueues(VariableListenerSupport.java:214)
    at org.optaplanner.core.impl.score.director.AbstractScoreDirector.triggerVariableListeners(AbstractScoreDirector.java:295)
    at org.optaplanner.core.impl.heuristic.move.AbstractMove.doMove(AbstractMove.java:37)
    at org.optaplanner.core.impl.heuristic.move.AbstractMove.doMove(AbstractMove.java:31)
    at org.optaplanner.core.impl.heuristic.move.CompositeMove.doMove(CompositeMove.java:109)
    at org.optaplanner.core.impl.heuristic.move.CompositeMove.doMove(CompositeMove.java:38)
    at org.optaplanner.core.impl.score.director.AbstractScoreDirector.doAndProcessMove(AbstractScoreDirector.java:223)
    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:71)
    at org.optaplanner.core.impl.solver.AbstractSolver.runPhases(AbstractSolver.java:99)
    at org.optaplanner.core.impl.solver.DefaultSolver.solve(DefaultSolver.java:160)
    at org.optaplanner.examples.common.business.SolutionBusiness.solve(SolutionBusiness.java:344)
    at org.optaplanner.examples.common.swingui.SolverAndPersistenceFrame$SolveWorker.doInBackground(SolverAndPersistenceFrame.java:369)
    at java.desktop/javax.swing.SwingWorker$1.call(SwingWorker.java:304)
    at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:317)
    at java.desktop/javax.swing.SwingWorker.run(SwingWorker.java:343)
    at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144)
    at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642)
    at java.base/java.lang.Thread.run(Thread.java:1623)

我试着调试代码，但找不到为什么它抛出这个异常。请帮助我。