[IZPACK-1136] NPE in trace mode when gathering traced variables on the first panel

On launching an GUI installer in trace mode by -DTRACE=true -DSTACKTRACE=true the installer fails with the following output:

```
Jul 17, 2014 11:55:10 AM SEVERE: Error when switching panel
java.lang.NullPointerException
        at com.izforge.izpack.installer.debugger.Debugger.getChangedVariables(Debugger.java:171)
        at com.izforge.izpack.installer.debugger.Debugger.debugVariables(Debugger.java:118)
        at com.izforge.izpack.installer.debugger.Debugger.switchPanel(Debugger.java:400)
        at com.izforge.izpack.installer.gui.InstallerFrame.switchPanel(InstallerFrame.java:551)
        at com.izforge.izpack.installer.gui.InstallerFrame$5.switchPanel(InstallerFrame.java:410)
        at com.izforge.izpack.installer.gui.IzPanels.switchPanel(IzPanels.java:128)
        at com.izforge.izpack.installer.gui.IzPanels.switchPanel(IzPanels.java:36)
        at com.izforge.izpack.installer.panel.AbstractPanels.switchPanel(AbstractPanels.java:496)
        at com.izforge.izpack.installer.panel.AbstractPanels.next(AbstractPanels.java:250)
        at com.izforge.izpack.installer.gui.DefaultNavigator.next(DefaultNavigator.java:345)
        at com.izforge.izpack.installer.gui.DefaultNavigator.next(DefaultNavigator.java:328)
        at com.izforge.izpack.installer.gui.InstallerFrame.navigateNext(InstallerFrame.java:891)
        at com.izforge.izpack.installer.gui.InstallerController$2.run(InstallerController.java:50)
        at com.izforge.izpack.installer.gui.InstallerController.run(InstallerController.java:64)
        at com.izforge.izpack.installer.gui.InstallerController.launchInstallation(InstallerController.java:44)
        at com.izforge.izpack.installer.bootstrap.InstallerGui$1.run(InstallerGui.java:60)
        at java.awt.event.InvocationEvent.dispatch(Unknown Source)
        at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
        at java.awt.EventQueue.access$400(Unknown Source)
        at java.awt.EventQueue$2.run(Unknown Source)
        at java.awt.EventQueue$2.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.security.AccessControlContext$1.doIntersectionPrivilege(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)

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
        at com.izforge.izpack.installer.gui.IzPanelView.validateDynamicConditions(IzPanelView.java:109)
        at com.izforge.izpack.installer.panel.AbstractPanelView.isValid(AbstractPanelView.java:244)
        at com.izforge.izpack.installer.gui.IzPanelView.isValid(IzPanelView.java:61)
        at com.izforge.izpack.installer.panel.AbstractPanels.executeValidationActions(AbstractPanels.java:545)
        at com.izforge.izpack.installer.panel.AbstractPanels.isValid(AbstractPanels.java:173)
        at com.izforge.izpack.installer.panel.AbstractPanels.next(AbstractPanels.java:245)
        at com.izforge.izpack.installer.gui.DefaultNavigator.next(DefaultNavigator.java:345)
        at com.izforge.izpack.installer.gui.DefaultNavigator.next(DefaultNavigator.java:328)
        at com.izforge.izpack.installer.gui.DefaultNavigator$NavigationHandler.navigate(DefaultNavigator.java:562)
        at com.izforge.izpack.installer.gui.DefaultNavigator$NavigationHandler.access$100(DefaultNavigator.java:525)
        at com.izforge.izpack.installer.gui.DefaultNavigator$NavigationHandler$1$1.run(DefaultNavigator.java:545)
        at java.awt.event.InvocationEvent.dispatch(Unknown Source)
        at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
        at java.awt.EventQueue.access$400(Unknown Source)
        at java.awt.EventQueue$2.run(Unknown Source)
        at java.awt.EventQueue$2.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.security.AccessControlContext$1.doIntersectionPrivilege(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)
```