PX Illegal Argument Exception for SetNotifier function. Please someone guide me to solve this error. I have pasted the code below.

public class SetNotifiersEvent implements IEventAction
 
 
 
{ 
public IRow row1; 
IUserGroup usergroup;
 
@Override 
public EventActionResult doAction(IAgileSession session, INode node, 
IEventInfo info) { 
try { 
//IChange change = null; 
IWFChangeStatusEventInfo changestatus = (IWFChangeStatusEventInfo) info; 
IChange change; 
String str1 = ""; 
//IItem itm; 
IDataObject obj = changestatus.getDataObject(); 
Collection al = new ArrayList(); 
if(obj instanceof IChange) 
{ 
change = (IChange) obj;
 
ITable table = change.getTable(ChangeConstants.TABLE_AFFECTEDITEMS); 
Iterator it = table.iterator(); 
while(it.hasNext()) 
{ 
IRow row = (IRow) it.next(); 
ICell attr = (ICell) row.getCell(ChangeConstants.ATT_AFFECTED_ITEMS_ITEM_TYPE);
 
String itemNumber = (String) row.getValue(ChangeConstants.ATT_AFFECTED_ITEMS_ITEM_NUMBER);
 
IAgileList str = (IAgileList) attr.getValue(); 
change.logAction(str.toString()); 
//IAgileList[] list1 = str.getSelection(); 
//change.logAction(list1.toString()); 
change.logAction("Inside while, Before IF3-"+str); 
//change.logAction("Inside while, Before IF2-"+str.getName()); 
//change.logAction("Inside while, Before IF1-"+str.getValue()); 
if((str.toString()).equals("Business Process (Document Type)")|| (str.toString()).equals("Product Definition (Document Type)")) 
{ 
change.logAction("Inside IF"); 
/*IItem item1= session.getObject(I, arg1)*/ 
change.logAction("ItemNumber"+itemNumber); 
IItem item = (IItem) session.getObject(ItemConstants.CLASS_DOCUMENT, 
itemNumber);
 
ICell cell =item.getCell(ItemConstants.ATT_PAGE_TWO_LIST12); 
IAgileList lis = (IAgileList) cell.getValue(); 
change.logAction("value="+lis.toString()); 
String li = lis.toString(); 
//} 
List usergroup1 = new ArrayList(); 
change.logAction("adding notifiers2"); 
IQuery query = (IQuery)session.createObject(-5, "User Group"); 
change.logAction("adding notifiers3"); 
query.setCriteria((new StringBuilder("[General Info.Name] contains 'R&D'")).toString());
 
/*query.setCriteria((new StringBuilder("[General Info.Name] equal to R&D_Test")).toString()); 
*/
 
ITable tbl = query.execute(); 
Iterator iter = tbl.iterator(); 
//ArrayList users = new ArrayList(); 
//IUser user; 
change.logAction("adding notifiers1");
 
for(; iter.hasNext(); usergroup1.add(usergroup)) 
{ change.logAction("adding notifiers5"+usergroup); 
usergroup = (IUserGroup)((IRow)iter.next()).getReferent(); 
} 
/*usergroup1.add("R&D_Test");*/
 
change.logAction("adding notifiers4="+usergroup1);
 
changestatus.setNotifiers(usergroup1); //Error_Line 
change.logAction("adding notifiers"); 
ActionResult result= new ActionResult(ActionResult.STRING,"Finished"); 
return new EventActionResult(info,result); 
}
 
}
 
}
 
//}
 
} catch (APIException e) { 
// TODO Auto-generated catch block 
e.printStackTrace(); 
ActionResult result= new ActionResult(-1,"Exception"); 
return new EventActionResult(info,result); 
}
 
ActionResult result= new ActionResult(ActionResult.STRING,"Success"); 
return new EventActionResult(info,result); 
} 
}