steps:
1, new a report library
2, go to outline and select theme
3, new theme and select report item theme
4, try to use the library for example right click on the library outline or reopen the library

actual results:
after right clicking on library layout, npe was thrown. and library can not be reopened anymore.
java.lang.classcastexception: org.eclipse.birt.report.model.api.reportitemthemehandle cannot be cast to org.eclipse.birt.report.model.api.themehandle
	at org.eclipse.birt.report.designer.internal.ui.editors.schematic.providers.schematiccontextmenuprovider.populateaddstyleaction(schematiccontextmenuprovider.java:854)
	at org.eclipse.birt.report.designer.internal.ui.editors.schematic.providers.schematiccontextmenuprovider.createstylemenu(schematiccontextmenuprovider.java:789)
	at org.eclipse.birt.report.designer.internal.ui.editors.schematic.providers.schematiccontextmenuprovider.buildcontextmenu(schematiccontextmenuprovider.java:308)
	at org.eclipse.gef.contextmenuprovider.menuabouttoshow(contextmenuprovider.java:63)
	at org.eclipse.jface.action.menumanager.fireabouttoshow(menumanager.java:338)
	at org.eclipse.jface.action.menumanager.handleabouttoshow(menumanager.java:469)
	at org.eclipse.jface.action.menumanager.access$1(menumanager.java:465)
	at org.eclipse.jface.action.menumanager$2.menushown(menumanager.java:491)
	at org.eclipse.swt.widgets.typedlistener.handleevent(typedlistener.java:241)
	at org.eclipse.swt.widgets.eventtable.sendevent(eventtable.java:84)
	at org.eclipse.swt.widgets.widget.sendevent(widget.java:1053)
	at org.eclipse.swt.widgets.widget.sendevent(widget.java:1077)
	at org.eclipse.swt.widgets.widget.sendevent(widget.java:1058)
	at org.eclipse.swt.widgets.control.wm_initmenupopup(control.java:4485)
	at org.eclipse.swt.widgets.control.windowproc(control.java:4188)
	at org.eclipse.swt.widgets.canvas.windowproc(canvas.java:341)
	at org.eclipse.swt.widgets.decorations.windowproc(decorations.java:1598)
	at org.eclipse.swt.widgets.shell.windowproc(shell.java:2038)
	at org.eclipse.swt.widgets.display.windowproc(display.java:4873)
	at org.eclipse.swt.internal.win32.os.trackpopupmenu(native method)
	at org.eclipse.swt.widgets.menu._setvisible(menu.java:256)
	at org.eclipse.swt.widgets.display.runpopups(display.java:4107)
	at org.eclipse.swt.widgets.display.readanddispatch(display.java:3651)
	at org.eclipse.ui.internal.workbench.runeventloop(workbench.java:2629)
	at org.eclipse.ui.internal.workbench.runui(workbench.java:2593)
	at org.eclipse.ui.internal.workbench.access$4(workbench.java:2427)
	at org.eclipse.ui.internal.workbench$7.run(workbench.java:670)
	at org.eclipse.core.databinding.observable.realm.runwithdefault(realm.java:332)
	at org.eclipse.ui.internal.workbench.createandrunworkbench(workbench.java:663)
	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)
tested on 2.6.0.v20100531-0630
fixed by re-tagging.
report item theme option is removed. verified in 2.6.0.v20100601-0630 build.