When port is null the following exception occurs

Exception occurred during event dispatching:
java.lang.NumberFormatException:
        at java.lang.Throwable.fillInStackTrace(Native Method)
        at java.lang.Throwable.fillInStackTrace(Compiled Code)
        at java.lang.Throwable.<init>(Compiled Code)
        at java.lang.Exception.<init>(Compiled Code)
        at java.lang.RuntimeException.<init>(RuntimeException.java:47)
        at java.lang.IllegalArgumentException.<init>(IllegalArgumentException.java:43)
        at java.lang.NumberFormatException.<init>(NumberFormatException.java:43)
        at java.lang.Integer.parseInt(Compiled Code)
        at java.lang.Integer.parseInt(Compiled Code)
        at com.sun.javaws.ui.prefs.GeneralPanel.apply(GeneralPanel.java:160)
        at com.sun.javaws.ui.prefs.UnixGeneralPanel.apply(UnixGeneralPanel.java:99)
        at com.sun.javaws.ui.general.AbstractController.apply(Compiled Code)
        at com.sun.javaws.ui.prefs.PrefsPanel.apply(PrefsPanel.java:155)
        at com.sun.javaws.ui.prefs.PrefsPanel.access$2(PrefsPanel.java:154)
        at com.sun.javaws.ui.prefs.PrefsPanel$3.actionPerformed(PrefsPanel.java:74)
        at javax.swing.AbstractButton.fireActionPerformed(Compiled Code)
        at javax.swing.AbstractButton$ForwardActionEvents.actionPerformed(Compiled Code)
        at javax.swing.DefaultButtonModel.fireActionPerformed(Compiled Code)
        at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:250)
        at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Compiled Code)
        at java.awt.Component.processMouseEvent(Compiled Code)
        at java.awt.Component.processEvent(Compiled Code)
        at java.awt.Container.processEvent(Compiled Code)
        at java.awt.Component.dispatchEventImpl(Compiled Code)
        at java.awt.Container.dispatchEventImpl(Compiled Code)
        at java.awt.Component.dispatchEvent(Compiled Code)
        at java.awt.LightweightDispatcher.retargetMouseEvent(Compiled Code)
        at java.awt.LightweightDispatcher.processMouseEvent(Compiled Code)
        at java.awt.LightweightDispatcher.dispatchEvent(Compiled Code)
        at java.awt.Container.dispatchEventImpl(Compiled Code)
        at java.awt.Window.dispatchEventImpl(Compiled Code)
        at java.awt.Component.dispatchEvent(Compiled Code)
        at java.awt.EventQueue.dispatchEvent(Compiled Code)
        at java.awt.EventDispatchThread.pumpOneEvent(Compiled Code)
        at java.awt.EventDispatchThread.pumpEvents(Compiled Code)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:83)