Please remove catch on WTException and throw as new WTException. It should work
wt.change2.WTChangeActivity2 ca = (wt.change2.WTChangeActivity2)primaryBusinessObject;
wt.fc.QueryResult objQueryResult = wt.change2.ChangeHelper2.service.getChangeablesAfter(ca);
while(objQueryResult.hasMoreElements()){
wt.enterprise.RevisionControlled obj = (wt.enterprise.RevisionControlled) objQueryResult.nextElement();
if(wt.vc.wip.WorkInProgressHelper.isCheckedOut(obj)){
result=false;
throw new wt.util.WTException("Please make sure all resulting objects are checked in before completing task");
}
}