Error During Simulation

Error during simulation.
I'm working on my final project and suddenly alice gives me this error!
can someone please help?

Alice version: 2.2 03/01/2012
os.name: Mac OS X
os.version: 10.7.4
os.arch: x86_64
java.vm.name: Java HotSpot(TM) 64-Bit Server VM
java.vm.version: 20.6-b01-415
user.dir: /Applications/Alice.app/Contents/Required

Throwable that caused the error:
java.lang.NullPointerException
at edu.cmu.cs.stage3.alice.core.question.DistanceTo.g etValue(DistanceTo.java:35)
at edu.cmu.cs.stage3.alice.core.question.SubjectObjec tQuestion.getValue(SubjectObjectQuestion.java:104)
at edu.cmu.cs.stage3.alice.core.question.SubjectQuest ion.getValue(SubjectQuestion.java:36)
at edu.cmu.cs.stage3.alice.core.Property.evaluateIfNe cessary(Property.java:381)
at edu.cmu.cs.stage3.alice.core.Property.getValue(Pro perty.java:408)
at edu.cmu.cs.stage3.alice.core.property.NumberProper ty.getNumberValue(NumberProperty.java:33)
at edu.cmu.cs.stage3.alice.core.property.NumberProper ty.doubleValue(NumberProperty.java:36)
at edu.cmu.cs.stage3.alice.core.property.NumberProper ty.doubleValue(NumberProperty.java:44)
at edu.cmu.cs.stage3.alice.core.question.BinaryNumber ResultingInBooleanQuestion.getValue(BinaryNumberRe sultingInBooleanQuestion.java:35)
at edu.cmu.cs.stage3.alice.core.Property.evaluateIfNe cessary(Property.java:381)
at edu.cmu.cs.stage3.alice.core.Property.getValue(Pro perty.java:408)
at edu.cmu.cs.stage3.alice.core.property.BooleanPrope rty.getValue(BooleanProperty.java:34)
at edu.cmu.cs.stage3.alice.core.property.BooleanPrope rty.getBooleanValue(BooleanProperty.java:46)
at edu.cmu.cs.stage3.alice.core.property.BooleanPrope rty.booleanValue(BooleanProperty.java:49)
at edu.cmu.cs.stage3.alice.core.behavior.ConditionalB ehavior.preSchedule(ConditionalBehavior.java:38)
at edu.cmu.cs.stage3.alice.core.Sandbox.scheduleBehav iors(Sandbox.java:69)
at edu.cmu.cs.stage3.alice.core.World.scheduleBehavio rs(World.java:458)
at edu.cmu.cs.stage3.alice.core.World.schedule(World. java:500)
at edu.cmu.cs.stage3.alice.core.clock.DefaultClock.sc hedule(DefaultClock.java:99)
at edu.cmu.cs.stage3.alice.authoringtool.AuthoringToo l$7.run(AuthoringTool.java:713)
at edu.cmu.cs.stage3.alice.authoringtool.util.Default Scheduler.simulateOnce(DefaultScheduler.java:115)
at edu.cmu.cs.stage3.alice.authoringtool.util.Default Scheduler.run(DefaultScheduler.java:76)
at edu.cmu.cs.stage3.scheduler.AbstractScheduler.run( AbstractScheduler.java:45)
at java.awt.event.InvocationEvent.dispatch(Invocation Event.java:209)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.j ava:682)
at java.awt.EventQueue.access$000(EventQueue.java:85)
at java.awt.EventQueue$1.run(EventQueue.java:643)
at java.awt.EventQueue$1.run(EventQueue.java:641)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectio nPrivilege(AccessControlContext.java:87)
at java.awt.EventQueue.dispatchEvent(EventQueue.java: 652)
at java.awt.EventDispatchThread.pumpOneEventForFilter s(EventDispatchThread.java:296)
at java.awt.EventDispatchThread.pumpEventsForFilter(E ventDispatchThread.java:211)
at java.awt.EventDispatchThread.pumpEventsForFilter(E ventDispatchThread.java:205)
at java.awt.Dialog$1.run(Dialog.java:1044)
at java.awt.Dialog$3.run(Dialog.java:1096)
at java.security.AccessController.doPrivileged(Native Method)
at java.awt.Dialog.show(Dialog.java:1094)
at java.awt.Component.show(Component.java:1584)
at java.awt.Component.setVisible(Component.java:1536)
at java.awt.Window.setVisible(Window.java:841)
at java.awt.Dialog.setVisible(Dialog.java:984)
at edu.cmu.cs.stage3.swing.DialogManager.showModalDia log(DialogManager.java:65)
at edu.cmu.cs.stage3.swing.DialogManager.showDialog(D ialogManager.java:135)
at edu.cmu.cs.stage3.alice.authoringtool.AuthoringToo l.play(AuthoringTool.java:5280)
at edu.cmu.cs.stage3.alice.authoringtool.Actions$16.a ctionPerformed(Actions.java:177)
at javax.swing.AbstractButton.fireActionPerformed(Abs tractButton.java:2028)
at javax.swing.AbstractButton$Handler.actionPerformed (AbstractButton.java:2351)
at javax.swing.DefaultButtonModel.fireActionPerformed (DefaultButtonModel.java:387)
at javax.swing.DefaultButtonModel.setPressed(DefaultB uttonModel.java:242)
at javax.swing.plaf.basic.BasicButtonListener.mouseRe leased(BasicButtonListener.java:236)
at java.awt.AWTEventMulticaster.mouseReleased(AWTEven tMulticaster.java:272)
at java.awt.Component.processMouseEvent(Component.jav a:6375)
at javax.swing.JComponent.processMouseEvent(JComponen t.java:3267)
at java.awt.Component.processEvent(Component.java:614 0)
at java.awt.Container.processEvent(Container.java:208 3)
at java.awt.Component.dispatchEventImpl(Component.jav a:4737)
at java.awt.Container.dispatchEventImpl(Container.jav a:2141)
at java.awt.Component.dispatchEvent(Component.java:45 65)
at java.awt.LightweightDispatcher.retargetMouseEvent( Container.java:4619)
at java.awt.LightweightDispatcher.processMouseEvent(C ontainer.java:4280)
at java.awt.LightweightDispatcher.dispatchEvent(Conta iner.java:4210)
at java.awt.Container.dispatchEventImpl(Container.jav a:2127)
at java.awt.Window.dispatchEventImpl(Window.java:2482 )
at java.awt.Component.dispatchEvent(Component.java:45 65)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.j ava:684)
at java.awt.EventQueue.access$000(EventQueue.java:85)
at java.awt.EventQueue$1.run(EventQueue.java:643)
at java.awt.EventQueue$1.run(EventQueue.java:641)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectio nPrivilege(AccessControlContext.java:87)
at java.security.AccessControlContext$1.doIntersectio nPrivilege(AccessControlContext.java:98)
at java.awt.EventQueue$2.run(EventQueue.java:657)
at java.awt.EventQueue$2.run(EventQueue.java:655)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectio nPrivilege(AccessControlContext.java:87)
at java.awt.EventQueue.dispatchEvent(EventQueue.java: 654)
at java.awt.EventDispatchThread.pumpOneEventForFilter s(EventDispatchThread.java:296)
at java.awt.EventDispatchThread.pumpEventsForFilter(E ventDispatchThread.java:211)
at java.awt.EventDispatchThread.pumpEventsForHierarch y(EventDispatchThread.java:201)
at java.awt.EventDispatchThread.pumpEvents(EventDispa tchThread.java:196)
at java.awt.EventDispatchThread.pumpEvents(EventDispa tchThread.java:188)
at java.awt.EventDispatchThread.run(EventDispatchThre ad.java:122)