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)

[jediaction]

These errors occur frequently with Alice. Try reverting to the backups. Sadly, there is not much else you can do unless you understand Python scripting. There is a way to load the files and erase the file that caused the error to be able to load the project but it's very advanced. Going ot backups is the best you can do.