How to fetch change set by work item link targetRe

问题:

First please let me know the difference between these two

1. com.ibm.team.filesystem.workitems.change_set
2. com.ibm.team.workitem.linktype.scm.tracksChanges

Because I have found some change set links using trackschanges Id and after that when I try to fetchCompleteItem for the link it return null or exception like that data is not present in the database. I am using the below code to fetchCompleteItem.

Really stuck here please help:

for(Object wI : links){
  ILink link = (ILink) wI;
  Object source = link.getTargetRef().resolve();
  IChangeSetHandle changeSetHandle = (IChangeSetHandle) link.getTargetRef().resolve();
  wiHandles.add((IChangeSetHandle) link.getTargetRef().resolve());

  if (source instanceof IChangeSetHandle) {
    changeSet = (IChangeSet) repo.itemManager().fetchCompleteItem(
      changeSetHandle,
      IItemManager.DEFAULT, monitor);
    System.out.println("changeset---1"+changeSet);
  }
}