In a new schema, drop an element.
Add a Choice to the element (1)
Add a Squence to the Choice (2)
Change Choice   (1) to All or Sequence -> ok
Change Sequence (2) to Choice or All   -> NPE below

NOTE: 
+ "change" either via double-click or Properties.
+ the NPE seems harmless... simply dismiss it and continue working in the IDE. 


java.lang.IllegalArgumentException: Exception during flush:
        at
org.netbeans.modules.xml.axi.impl.ModelAccessImpl.flush(ModelAccessImpl.java:127)
        at
org.netbeans.modules.xml.xam.AbstractModel.endTransaction(AbstractModel.java:260)
        at
org.netbeans.modules.xml.schema.abe.CompositorPanel.addNewCompositorAt(CompositorPanel.java:441)
        at
org.netbeans.modules.xml.schema.abe.CompositorPanel.tweenerDrop(CompositorPanel.java:489)
        at
org.netbeans.modules.xml.schema.abe.ContainerPanel$1.drop(ContainerPanel.java:324)
        at
org.netbeans.modules.xml.schema.abe.TweenerPanel.drop(TweenerPanel.java:228)
        at
org.netbeans.modules.xml.schema.abe.ABEBaseDropPanel$1.drop(ABEBaseDropPanel.java:77)
        at java.awt.dnd.DropTarget.drop(DropTarget.java:430)
        at
sun.awt.dnd.SunDropTargetContextPeer.processDropMessage(SunDropTargetContextPeer.java:500)
        at
sun.awt.dnd.SunDropTargetContextPeer$EventDispatcher.dispatchDropEvent(SunDropTargetContextPeer.java:812)
        at
sun.awt.dnd.SunDropTargetContextPeer$EventDispatcher.dispatchEvent(SunDropTargetContextPeer.java:736)
        at
sun.awt.dnd.SunDropTargetEvent.dispatch(SunDropTargetEvent.java:30)
        at java.awt.Component.dispatchEventImpl(Component.java:4222)
        at java.awt.Container.dispatchEventImpl(Container.java:2018)
        at java.awt.Component.dispatchEvent(Component.java:4195)
        at
java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4222)
        at
java.awt.LightweightDispatcher.processDropTargetEvent(Container.java:3957)
        at java.awt.LightweightDispatcher.dispatchEvent(Container.java:3811)
        at java.awt.Container.dispatchEventImpl(Container.java:2004)
        at java.awt.Window.dispatchEventImpl(Window.java:2210)
        at java.awt.Component.dispatchEvent(Component.java:4195)
[catch] at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
        at
java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:273)
        at
java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:183)
        at
java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:173)
        at
java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:168)
        at
java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:160)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:121)
Caused by: java.lang.NullPointerException
        at
org.netbeans.modules.xml.xam.AbstractComponent.verifyWrite(AbstractComponent.java:188)
        at
org.netbeans.modules.xml.xam.AbstractComponent.appendChild(AbstractComponent.java:302)
        at
org.netbeans.modules.xml.schema.model.impl.ChoiceImpl.addSequence(ChoiceImpl.java:92)
        at
org.netbeans.modules.xml.axi.impl.Util.createSequence(Util.java:517)
        at
org.netbeans.modules.xml.axi.impl.DefaultSchemaGenerator.visit(DefaultSchemaGenerator.java:298)
        at
org.netbeans.modules.xml.axi.Compositor.accept(Compositor.java:93)
        at
org.netbeans.modules.xml.axi.impl.DefaultSchemaGenerator.addSchemaComponent(DefaultSchemaGenerator.java:413)
        at
org.netbeans.modules.xml.axi.impl.DefaultSchemaGenerator.updateSchema(DefaultSchemaGenerator.java:87)
        at
org.netbeans.modules.xml.axi.impl.SchemaGeneratorFactoryImpl.updateSchema(SchemaGeneratorFactoryImpl.java:61)
        at
org.netbeans.modules.xml.axi.impl.ModelAccessImpl.flush(ModelAccessImpl.java:125)
        ... 27 more