This code throws exception 
(org.netbeans.modules.web.webdata.WebApplicationImpl:94):

new AppChangeEvent(DataChangeEvent.CHANGED_EVENT, AppChangeEvent.WEBMODULE, 
this);

Problem is there 
(org.netbeans.modules.j2ee.server.datamodel.DataChangeEvent:23):

    public DataChangeEvent(int e,StandardData d) {
        // PENDING - get the FileObject from the data's server resource, if any.
        super(null);
        eventType = e;
        data = d;
    }

Exception:

Mon Apr 30 13:36:44 GMT+02:00 2001: 
org.openide.explorer.propertysheet.PropertyDetails$ExceptionHack: 
java.lang.IllegalArgumentException: null source
        at java.util.EventObject.<init>(EventObject.java:37)
        at org.openide.filesystems.FileEvent.<init>(FileEvent.java:52)
        at org.openide.filesystems.FileEvent.<init>(FileEvent.java:38)
        at org.netbeans.modules.j2ee.server.datamodel.DataChangeEvent.<init>
(DataChangeEvent.java:25)
        at org.netbeans.modules.j2ee.server.datamodel.AppChangeEvent.<init>
(AppChangeEvent.java:21)
        at 
org.netbeans.modules.web.webdata.WebApplicationImpl.fireWebModuleChange
(WebApplicationImpl.java:94)
        at org.netbeans.modules.web.webdata.WebApplicationImpl.propertyChange
(WebApplicationImpl.java:88)
        at org.openide.loaders.DataObject.firePropertyChange
(DataObject.java:743)
        at org.netbeans.modules.web.context.WebInfObject.access$301
(WebInfObject.java:44)
        at org.netbeans.modules.web.context.WebInfObject$1.setValue
(WebInfObject.java:235)
        at org.openide.explorer.propertysheet.PropertyDetails.setPropertyValue
(PropertyDetails.java:185)
        at org.openide.explorer.propertysheet.PropertyDisplayer.setPropertyValue
(PropertyDisplayer.java:830)
        at org.openide.explorer.propertysheet.PropertyDisplayer.setAsText
(PropertyDisplayer.java:799)
        at 
org.openide.explorer.propertysheet.PropertyDisplayer$WriteComponentListener.chan
geValue(PropertyDisplayer.java:1088)
        at org.openide.explorer.propertysheet.PropertyDisplayer.setReadState
(PropertyDisplayer.java:354)
        at org.openide.explorer.propertysheet.PropertyDisplayer.setAsText
(PropertyDisplayer.java:792)
        at 
org.openide.explorer.propertysheet.PropertyDisplayer$WriteComponentListener.acti
onPerformed(PropertyDisplayer.java:1054)
        at javax.swing.JTextField.fireActionPerformed(JTextField.java:421)
        at javax.swing.JTextField.postActionEvent(JTextField.java:586)
        at javax.swing.JTextField$NotifyAction.actionPerformed
(JTextField.java:696)
        at javax.swing.SwingUtilities.notifyAction(SwingUtilities.java:1702)
        at javax.swing.JComponent.processKeyBinding(JComponent.java:2156)
        at javax.swing.JComponent.processKeyBindings(JComponent.java:2182)
        at javax.swing.JComponent.processKeyEvent(JComponent.java:2119)
        at java.awt.Component.processEvent(Component.java:3555)
        at java.awt.Container.processEvent(Container.java:1164)
        at java.awt.Component.dispatchEventImpl(Component.java:2595)
        at java.awt.Container.dispatchEventImpl(Container.java:1213)
        at java.awt.Component.dispatchEvent(Component.java:2499)
        at java.awt.LightweightDispatcher.processKeyEvent(Container.java:2155)
        at java.awt.LightweightDispatcher.dispatchEvent(Container.java:2135)
        at java.awt.Container.dispatchEventImpl(Container.java:1200)
        at java.awt.Window.dispatchEventImpl(Window.java:912)
        at java.awt.Component.dispatchEvent(Component.java:2499)
[catch] at java.awt.EventQueue.dispatchEvent(EventQueue.java:319)
        at java.awt.EventDispatchThread.pumpOneEvent
(EventDispatchThread.java:103)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:84)