nullpointer Exception when setting category to null

Hi,

I have a plugin that deletes all information from workitem and setting it's type to some custom "archived" type.
All is working fine, except when setting the category to null.
I have tried setting to "unassigned" too, same behavior.

It looks like that:
IWorkItem workItem = wc.getWorkItem();
workItem.setOwner(contributorManager
.fetchContributorByUserId("unassigned", null));
workItem.setTarget(null);
// workItem.setCategory(null);
IAttribute attrType = workItemClient.findAttribute(
projectArea, IWorkItem.TYPE_PROPERTY, null);
IWorkItemType zArchivedType = workItemCommon
.findWorkItemType(projectArea, "archived", null);
workItem.setValue(attrType, zArchivedType.getIdentifier());
IWorkItemReferences references = wc.getReferences();
List<IEndPointDescriptor> refTypes = references.getTypes();
for (IEndPointDescriptor endPointDescriptor : refTypes) {
List<IReference> refList = references
.getReferences(endPointDescriptor);
for (IReference reference : refList) {
references.remove(reference);
}
}
IDetailedStatus status = wc.save(null);