Error during simulation !!

Hi,

I received an error while running my project on Alice.

The project is about a guy walks while pressing the arrow keys.
The problem seems to happen after I stop pressing the up arrow key which makes the guy stop walking forward.




Here is the error message:

Error during simulation.


Alice version: 2.2  09/10/2010

Throwable that caused the error:
java.lang.NullPointerException
	at edu.cmu.cs.stage3.alice.core.Behavior$RuntimeStack.pop(Behavior.java:49)
	at edu.cmu.cs.stage3.alice.core.Behavior.closeFork(Behavior.java:183)
	at edu.cmu.cs.stage3.alice.core.response.DoTogether$RuntimeDoTogether.epilogue(DoTogether.java:89)
	at edu.cmu.cs.stage3.alice.core.response.CompositeResponse$RuntimeCompositeResponse.childrenEpiloguesIfNecessary(CompositeResponse.java:67)
	at edu.cmu.cs.stage3.alice.core.response.CompositeResponse$RuntimeCompositeResponse.epilogue(CompositeResponse.java:74)
	at edu.cmu.cs.stage3.alice.core.response.CallToUserDefinedResponse$RuntimeCallToUserDefinedResponse.epilogue(CallToUserDefinedResponse.java:98)
	at edu.cmu.cs.stage3.alice.core.behavior.AbstractConditionalBehavior.internalSchedule(AbstractConditionalBehavior.java:114)
	at edu.cmu.cs.stage3.alice.core.Behavior.schedule(Behavior.java:233)
	at edu.cmu.cs.stage3.alice.core.Sandbox.scheduleBehaviors(Sandbox.java:70)
	at edu.cmu.cs.stage3.alice.core.World.scheduleBehaviors(World.java:457)
	at edu.cmu.cs.stage3.alice.core.World.schedule(World.java:499)
	at edu.cmu.cs.stage3.alice.core.clock.DefaultClock.schedule(DefaultClock.java:99)
	at edu.cmu.cs.stage3.alice.authoringtool.AuthoringTool$7.run(AuthoringTool.java:693)
	at edu.cmu.cs.stage3.alice.authoringtool.util.DefaultScheduler.simulateOnce(DefaultScheduler.java:115)
	at edu.cmu.cs.stage3.alice.authoringtool.util.DefaultScheduler.run(DefaultScheduler.java:76)
	at edu.cmu.cs.stage3.scheduler.AbstractScheduler.run(AbstractScheduler.java:45)
	at java.awt.event.InvocationEvent.dispatch(Unknown Source)
	at java.awt.EventQueue.dispatchEvent(Unknown Source)
	at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
	at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
	at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
	at java.awt.Dialog$1.run(Unknown Source)
	at java.awt.Dialog$3.run(Unknown Source)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.awt.Dialog.show(Unknown Source)
	at java.awt.Component.show(Unknown Source)
	at java.awt.Component.setVisible(Unknown Source)
	at java.awt.Window.setVisible(Unknown Source)
	at java.awt.Dialog.setVisible(Unknown Source)
	at edu.cmu.cs.stage3.swing.DialogManager.showModalDialog(DialogManager.java:61)
	at edu.cmu.cs.stage3.swing.DialogManager.showDialog(DialogManager.java:131)
	at edu.cmu.cs.stage3.alice.authoringtool.AuthoringTool.play(AuthoringTool.java:5165)
	at edu.cmu.cs.stage3.alice.authoringtool.Actions$16.actionPerformed(Actions.java:177)
	at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
	at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
	at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
	at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
	at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
	at java.awt.AWTEventMulticaster.mouseReleased(Unknown Source)
	at java.awt.Component.processMouseEvent(Unknown Source)
	at javax.swing.JComponent.processMouseEvent(Unknown Source)
	at java.awt.Component.processEvent(Unknown Source)
	at java.awt.Container.processEvent(Unknown Source)
	at java.awt.Component.dispatchEventImpl(Unknown Source)
	at java.awt.Container.dispatchEventImpl(Unknown Source)
	at java.awt.Component.dispatchEvent(Unknown Source)
	at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
	at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
	at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
	at java.awt.Container.dispatchEventImpl(Unknown Source)
	at java.awt.Window.dispatchEventImpl(Unknown Source)
	at java.awt.Component.dispatchEvent(Unknown Source)
	at java.awt.EventQueue.dispatchEvent(Unknown Source)
	at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
	at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
	at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
	at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
	at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
	at java.awt.EventDispatchThread.run(Unknown Source)

If any one has an idea about it please help as soon as possible, I must get this project done by next Monday !



Thank you...