Fix : NPE in ListPropertyEditorPresentation after creating a Java type

TablePropertyEditorPresentation fails to remove action handlers that it
registers for Delete, Move Up and Move Down actions.

1. Create sapphire-gallery.xml in a Java project
2. Navigate to "List Properties" -> "Homogeneous With Java Types"
3. Use the add button on the table property editor to add a new item
4. Enter class name "Foo" through the table cell editor
5. De-activate and re-activate the cell editor
6. Use the cell editor's create action to create the class Foo
7. After the created class is opened, switch back to the gallery editor
8. Switch to another node in the content outline
9. Come back to the original table property editor and add another item

java.lang.NullPointerException
	at org.eclipse.sapphire.ui.forms.swt.PropertyEditorPresentation.property(PropertyEditorPresentation.java:122)
	at org.eclipse.sapphire.ui.forms.swt.ListPropertyEditorPresentation.property(ListPropertyEditorPresentation.java:76)
	at org.eclipse.sapphire.ui.forms.swt.ListPropertyEditorPresentation.list(ListPropertyEditorPresentation.java:81)
	at org.eclipse.sapphire.ui.forms.swt.TablePropertyEditorPresentation$MoveUpActionHandler.computeEnablementState(TablePropertyEditorPresentation.java:2289)
	at org.eclipse.sapphire.ui.forms.PropertyEditorActionHandler$3.run(PropertyEditorActionHandler.java:83)
	at org.eclipse.sapphire.ui.forms.swt.SwtUtil.runOnDisplayThread(SwtUtil.java:288)
	at org.eclipse.sapphire.ui.forms.swt.SwtUtil.runOnDisplayThread(SwtUtil.java:304)
	at org.eclipse.sapphire.ui.forms.PropertyEditorActionHandler.refreshEnablementState(PropertyEditorActionHandler.java:87)
	at org.eclipse.sapphire.ui.forms.PropertyEditorActionHandler$1.handleTypedEvent(PropertyEditorActionHandler.java:45)
	at org.eclipse.sapphire.ui.forms.PropertyEditorActionHandler$1.handleTypedEvent(PropertyEditorActionHandler.java:1)
	at org.eclipse.sapphire.FilteredListener.handle(FilteredListener.java:35)
	at org.eclipse.sapphire.EventDeliveryJob.run(EventDeliveryJob.java:52)
	at org.eclipse.sapphire.JobQueue.process(JobQueue.java:94)
	at org.eclipse.sapphire.ListenerContext.broadcast(ListenerContext.java:126)
	at org.eclipse.sapphire.ListenerContext.broadcast(ListenerContext.java:132)
	at org.eclipse.sapphire.ui.SapphirePart.broadcast(SapphirePart.java:560)
	at org.eclipse.sapphire.ui.SapphirePart.initialize(SapphirePart.java:210)
	at org.eclipse.sapphire.ui.forms.MasterDetailsContentNodePart.init(MasterDetailsContentNodePart.java:271)
	at org.eclipse.sapphire.ui.SapphirePart.initialize(SapphirePart.java:206)
	at org.eclipse.sapphire.ui.forms.MasterDetailsContentNodePart$NodeFactory.nodes(MasterDetailsContentNodePart.java:1073)
	at org.eclipse.sapphire.ui.forms.MasterDetailsContentNodePart.refreshNodes(MasterDetailsContentNodePart.java:689)
	at org.eclipse.sapphire.ui.forms.MasterDetailsContentNodePart.access$0(MasterDetailsContentNodePart.java:677)
	at org.eclipse.sapphire.ui.forms.MasterDetailsContentNodePart$NodeFactory$1.handle(MasterDetailsContentNodePart.java:894)
	at org.eclipse.sapphire.EventDeliveryJob.run(EventDeliveryJob.java:52)
	at org.eclipse.sapphire.JobQueue.process(JobQueue.java:94)
	at org.eclipse.sapphire.ListenerContext.broadcast(ListenerContext.java:126)
	at org.eclipse.sapphire.ListenerContext.broadcast(ListenerContext.java:132)
	at org.eclipse.sapphire.Property.broadcast(Property.java:780)
	at org.eclipse.sapphire.ElementList.refreshContent(ElementList.java:214)
	at org.eclipse.sapphire.ElementList.refresh(ElementList.java:82)
	at org.eclipse.sapphire.ElementList.insert$(ElementList.java:389)
	at org.eclipse.sapphire.ElementList.insert(ElementList.java:339)
	at org.eclipse.sapphire.ui.forms.swt.TablePropertyEditorPresentation$AddActionHandler.executeTablePropertyEditorAction(TablePropertyEditorPresentation.java:2198)
	at org.eclipse.sapphire.ui.forms.swt.TablePropertyEditorPresentation$TablePropertyEditorActionHandler.run(TablePropertyEditorPresentation.java:2161)
	at org.eclipse.sapphire.ui.SapphireActionHandler.execute(SapphireActionHandler.java:95)
	at org.eclipse.sapphire.ui.forms.swt.SapphireToolBarActionPresentation$1.widgetSelected(SapphireToolBarActionPresentation.java:113)
	at org.eclipse.swt.widgets.TypedListener.handleEvent(TypedListener.java:248)
	at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:84)
	at org.eclipse.swt.widgets.Display.sendEvent(Display.java:4353)
	at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1061)
	at org.eclipse.swt.widgets.Display.runDeferredEvents(Display.java:4172)
	at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3761)
	at org.eclipse.e4.ui.internal.workbench.swt.PartRenderingEngine$9.run(PartRenderingEngine.java:1151)
	at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:337)
	at org.eclipse.e4.ui.internal.workbench.swt.PartRenderingEngine.run(PartRenderingEngine.java:1032)
	at org.eclipse.e4.ui.internal.workbench.E4Workbench.createAndRunUI(E4Workbench.java:148)
	at org.eclipse.ui.internal.Workbench$5.run(Workbench.java:638)
	at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:337)
	at org.eclipse.ui.internal.Workbench.createAndRunWorkbench(Workbench.java:582)
	at org.eclipse.ui.PlatformUI.createAndRunWorkbench(PlatformUI.java:150)
	at org.eclipse.ui.internal.ide.application.IDEApplication.start(IDEApplication.java:138)
	at org.eclipse.equinox.internal.app.EclipseAppHandle.run(EclipseAppHandle.java:196)
	at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.runApplication(EclipseAppLauncher.java:134)
	at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.start(EclipseAppLauncher.java:104)
	at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:380)
	at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:235)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:483)
	at org.eclipse.equinox.launcher.Main.invokeFramework(Main.java:648)
	at org.eclipse.equinox.launcher.Main.basicRun(Main.java:603)
	at org.eclipse.equinox.launcher.Main.run(Main.java:1465)
	at org.eclipse.equinox.launcher.Main.main(Main.java:1438)

Bugzilla: https://bugs.eclipse.org/bugs/show_bug.cgi?id=447470