public void addWorkingSetDescriptor(WorkingSetDescriptor descriptor) {
Assert.isTrue(!workingSetDescriptors.containsValue(descriptor),
"working set descriptor already registered"); //$NON-NLS-1$
IExtensionTracker tracker = PlatformUI.getWorkbench()
.getExtensionTracker();
tracker.registerObject(descriptor.getConfigurationElement()
.getDeclaringExtension(), descriptor,
IExtensionTracker.REF_WEAK);
workingSetDescriptors.put(descriptor.getId(), descriptor);
}