Creating a link between Parent Part and Child Part using windchill API's

Hi,

I am getting the following error while creating a link between a checked out working copy of a part and a child part.

(wt.vc.wip.wipResource/0) wt.vc.wip.WorkInProgressException: The object was improperly initialized (i.e., it was null).

at wt.method.RemoteMethodServer.invoke(RemoteMethodServer.java:795)

at wt.services.ServiceFactory$ClientInvocationHandler.invoke(ServiceFact

ory.java:349)

at $Proxy8.checkout(Unknown Source)

at com.injector.Injector_Test.main(Injector_Test.java:201)

Any suggestions on how to solve this error ?

Code for reference :

WTPart part = WTPart.newWTPart(

"Part1", "MyPart");

PersistenceHelper.

manager.save(part);

WTPart childPart = WTPart.newWTPart(

"CPart1", "MyChildPart");

PersistenceHelper.

manager.save(childPart);

WTPart Workingcopy = (WTPart)WorkInProgressHelper.

service.checkout(part, null, null).getWorkingCopy();

PersistenceHelper.

manager.save(link);

WorkInProgressHelper.

service.checkin(Workingcopy,null);

WTPartUsageLink link = WTPartUsageLink.newWTPartUsageLink(Workingcopy, (WTPartMaster) childPart.getMaster());