in Seam perspective:
1) open some xhtml in VPE;
2) open same xhtml in WPE (Web Page Editor);
3) restart Eclipse:

Unable to create editor ID org.eclipse.jst.pagedesigner.PageDesignerEditor: An unexpected exception was thrown.

java.lang.NullPointerException
at org.eclipse.jst.j2ee.componentcore.J2EEModuleVirtualComponent.getReferences(J2EEModuleVirtualComponent.java:176)
at org.eclipse.jst.j2ee.internal.web.classpath.WebAppLibrariesContainer.computeReferences(WebAppLibrariesContainer.java:100)
at org.eclipse.jst.common.jdt.internal.classpath.FlexibleProjectContainer.computeClasspathEntries(FlexibleProjectContainer.java:183)
at org.eclipse.jst.j2ee.internal.web.classpath.WebAppLibrariesContainer.computeClasspathEntries(WebAppLibrariesContainer.java:119)
at org.eclipse.jst.common.jdt.internal.classpath.FlexibleProjectContainer.<init>(FlexibleProjectContainer.java:120)
at org.eclipse.jst.j2ee.internal.web.classpath.WebAppLibrariesContainer.<init>(WebAppLibrariesContainer.java:58)
at org.eclipse.jst.j2ee.internal.web.classpath.WebAppLibrariesContainerInitializer.initialize(WebAppLibrariesContainerInitializer.java:35)
at org.eclipse.jdt.internal.core.JavaModelManager.initializeContainer(JavaModelManager.java:2707)
at org.eclipse.jdt.internal.core.JavaModelManager.getClasspathContainer(JavaModelManager.java:1850)
at org.eclipse.jdt.core.JavaCore.getClasspathContainer(JavaCore.java:2705)
at org.eclipse.jdt.internal.core.JavaProject.resolveClasspath(JavaProject.java:2645)
at org.eclipse.jdt.internal.core.JavaProject.resolveClasspath(JavaProject.java:2783)
at org.eclipse.jdt.internal.core.JavaProject.getResolvedClasspath(JavaProject.java:1948)
at org.eclipse.jst.jsp.core.taglib.ProjectDescription.ensureUpTodate(ProjectDescription.java:994)
at org.eclipse.jst.jsp.core.taglib.ProjectDescription.getAvailableTaglibRecords(ProjectDescription.java:1085)
at org.eclipse.jst.jsp.core.taglib.TaglibIndex.internalGetAvailableTaglibRecords(TaglibIndex.java:847)
at org.eclipse.jst.jsp.core.taglib.TaglibIndex.getAvailableTaglibRecords(TaglibIndex.java:506)
at org.eclipse.jst.jsf.core.metadata.internal.TaglibMetaDataLocator.locateMetaDataModelProviders(TaglibMetaDataLocator.java:69)
at org.eclipse.jst.jsf.common.metadata.internal.DomainLoadingStrategy.locateMetaDataSourceInstances(DomainLoadingStrategy.java:143)
at org.eclipse.jst.jsf.common.metadata.internal.DomainLoadingStrategy.load(DomainLoadingStrategy.java:55)
at org.eclipse.jst.jsf.common.metadata.internal.MetaDataModel.load(MetaDataModel.java:93)
at org.eclipse.jst.jsf.common.metadata.internal.MetaDataModelManager.loadMetadata(MetaDataModelManager.java:147)
at org.eclipse.jst.jsf.common.metadata.internal.MetaDataModelManager.getModel(MetaDataModelManager.java:90)
at org.eclipse.jst.jsf.common.metadata.query.TaglibDomainMetaDataQueryHelper.getMDModel(TaglibDomainMetaDataQueryHelper.java:205)
at org.eclipse.jst.jsf.common.metadata.query.TaglibDomainMetaDataQueryHelper.getModel(TaglibDomainMetaDataQueryHelper.java:90)
at org.eclipse.jst.jsf.common.metadata.query.TaglibDomainMetaDataQueryHelper.getEntities(TaglibDomainMetaDataQueryHelper.java:123)
at org.eclipse.jst.jsf.common.metadata.query.TaglibDomainMetaDataQueryHelper.getEntity(TaglibDomainMetaDataQueryHelper.java:101)
at org.eclipse.jst.jsf.common.metadata.query.TaglibDomainMetaDataQueryHelper.getTrait(TaglibDomainMetaDataQueryHelper.java:254)
at org.eclipse.jst.pagedesigner.dtmanager.DefaultDTInfoFactory.getDTInfo(DefaultDTInfoFactory.java:49)
at org.eclipse.jst.pagedesigner.dtmanager.DTManager.getDTInfo(DTManager.java:99)
at org.eclipse.jst.pagedesigner.dtmanager.converter.internal.DTTagConverterFactory.createConverter(DTTagConverterFactory.java:32)
at org.eclipse.jst.pagedesigner.dtmanager.DTManager.getTagConverter(DTManager.java:65)
at org.eclipse.jst.pagedesigner.parts.ElementEditPart.getTagConverter(ElementEditPart.java:163)
at org.eclipse.jst.pagedesigner.parts.ElementEditPart.setModel(ElementEditPart.java:84)
at org.eclipse.jst.pagedesigner.parts.HTMLEditPartsFactory.createEditPart(HTMLEditPartsFactory.java:69)
at org.eclipse.gef.editparts.AbstractEditPart.createChild(AbstractEditPart.java:268)
at org.eclipse.gef.editparts.AbstractEditPart.refreshChildren(AbstractEditPart.java:770)
at org.eclipse.jst.pagedesigner.parts.DocumentEditPart.refreshChildren(DocumentEditPart.java:134)
at org.eclipse.jst.pagedesigner.parts.DocumentEditPart.refreshChildren(DocumentEditPart.java:126)
at org.eclipse.gef.editparts.AbstractEditPart.refresh(AbstractEditPart.java:718)
at org.eclipse.gef.editparts.AbstractGraphicalEditPart.refresh(AbstractGraphicalEditPart.java:633)
at org.eclipse.jst.pagedesigner.parts.DocumentEditPart.refresh(DocumentEditPart.java:114)
at org.eclipse.gef.editparts.AbstractEditPart.addNotify(AbstractEditPart.java:252)
at org.eclipse.gef.editparts.AbstractGraphicalEditPart.addNotify(AbstractGraphicalEditPart.java:223)
at org.eclipse.jst.pagedesigner.parts.NodeEditPart.addNotify(NodeEditPart.java:104)
at org.eclipse.gef.editparts.AbstractEditPart.addChild(AbstractEditPart.java:211)
at org.eclipse.gef.editparts.SimpleRootEditPart.setContents(SimpleRootEditPart.java:105)
at org.eclipse.gef.ui.parts.AbstractEditPartViewer.setContents(AbstractEditPartViewer.java:617)
at org.eclipse.gef.ui.parts.AbstractEditPartViewer.setContents(AbstractEditPartViewer.java:626)
at org.eclipse.jst.pagedesigner.editors.SimpleGraphicalEditor.setModel(SimpleGraphicalEditor.java:425)
at org.eclipse.jst.pagedesigner.editors.HTMLEditor.connectDesignPage(HTMLEditor.java:363)
at org.eclipse.jst.pagedesigner.editors.HTMLEditor.createPages(HTMLEditor.java:443)
at org.eclipse.ui.part.MultiPageEditorPart.createPartControl(MultiPageEditorPart.java:348)
at org.eclipse.ui.internal.EditorReference.createPartHelper(EditorReference.java:670)
at org.eclipse.ui.internal.EditorReference.createPart(EditorReference.java:465)
at org.eclipse.ui.internal.WorkbenchPartReference.getPart(WorkbenchPartReference.java:595)
at org.eclipse.ui.internal.EditorAreaHelper.setVisibleEditor(EditorAreaHelper.java:271)
at org.eclipse.ui.internal.EditorManager.setVisibleEditor(EditorManager.java:1429)
at org.eclipse.ui.internal.EditorManager$5.runWithException(EditorManager.java:942)
at org.eclipse.ui.internal.StartupThreading$StartupRunnable.run(StartupThreading.java:31)
at org.eclipse.swt.widgets.RunnableLock.run(RunnableLock.java:35)
at org.eclipse.swt.widgets.Synchronizer.runAsyncMessages(Synchronizer.java:134)
at org.eclipse.swt.widgets.Display.runAsyncMessages(Display.java:4041)
at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3660)
at org.eclipse.ui.application.WorkbenchAdvisor.openWindows(WorkbenchAdvisor.java:803)
at org.eclipse.ui.internal.Workbench$31.runWithException(Workbench.java:1567)
at org.eclipse.ui.internal.StartupThreading$StartupRunnable.run(StartupThreading.java:31)
at org.eclipse.swt.widgets.RunnableLock.run(RunnableLock.java:35)
at org.eclipse.swt.widgets.Synchronizer.runAsyncMessages(Synchronizer.java:134)
at org.eclipse.swt.widgets.Display.runAsyncMessages(Display.java:4041)
at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3660)
at org.eclipse.ui.internal.Workbench.runUI(Workbench.java:2548)
at org.eclipse.ui.internal.Workbench.access$4(Workbench.java:2438)
at org.eclipse.ui.internal.Workbench$7.run(Workbench.java:671)
at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:332)
at org.eclipse.ui.internal.Workbench.createAndRunWorkbench(Workbench.java:664)
at org.eclipse.ui.PlatformUI.createAndRunWorkbench(PlatformUI.java:149)
at org.eclipse.ui.internal.ide.application.IDEApplication.start(IDEApplication.java:115)
at org.eclipse.equinox.internal.app.EclipseAppHandle.run(EclipseAppHandle.java:196)
at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.runApplication(EclipseAppLauncher.java:110)
at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.start(EclipseAppLauncher.java:79)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:369)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:179)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.eclipse.equinox.launcher.Main.invokeFramework(Main.java:619)
at org.eclipse.equinox.launcher.Main.basicRun(Main.java:574)
at org.eclipse.equinox.launcher.Main.run(Main.java:1407)