<field type="rule" align="right" variable="rmiport.value">
<spec txt=" RMIPort:" layout="N:4:4" />
<validator
class="com.izforge.izpack.util.PortValidator"
txt="Port is in use">
</validator>
</field>

[java] Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
[java] at com.izforge.izpack.panels.UserInputPanel.showMessageDialog(UserInputPanel.java:3581)
[java] at com.izforge.izpack.panels.UserInputPanel.showWarningMessageDialog(UserInputPanel.java:3598)
[java] at com.izforge.izpack.panels.UserInputPanel.readRuleField(UserInputPanel.java:1454)
[java] at com.izforge.izpack.panels.UserInputPanel.readInput(UserInputPanel.java:996)
[java] at com.izforge.izpack.panels.UserInputPanel.isValidated(UserInputPanel.java:838)
[java] at com.izforge.izpack.installer.InstallerFrame.navigateNext(InstallerFrame.java:1552)
[java] at com.izforge.izpack.installer.InstallerFrame.navigateNext(InstallerFrame.java:1543)
[java] at com.izforge.izpack.installer.InstallerFrame$NavigationHandler.actionPerformed(InstallerFrame.java:1626)
[java] at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
[java] at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
[java] at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
[java] at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
[java] at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
[java] at java.awt.Component.processMouseEvent(Unknown Source)
[java] at javax.swing.JComponent.processMouseEvent(Unknown Source)
[java] at java.awt.Component.processEvent(Unknown Source)
[java] at java.awt.Container.processEvent(Unknown Source)
[java] at java.awt.Component.dispatchEventImpl(Unknown Source)
[java] at java.awt.Container.dispatchEventImpl(Unknown Source)
[java] at java.awt.Component.dispatchEvent(Unknown Source)
[java] at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
[java] at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
[java] at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
[java] at java.awt.Container.dispatchEventImpl(Unknown Source)
[java] at java.awt.Window.dispatchEventImpl(Unknown Source)
[java] at java.awt.Component.dispatchEvent(Unknown Source)
[java] at java.awt.EventQueue.dispatchEvent(Unknown Source)
[java] at java.awt.EventDispatchThread.pumpOneEventForHierarchy(Unknown Source)
[java] at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
[java] at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
[java] at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
[java] at java.awt.EventDispatchThread.run(Unknown Source)