Is it compulsory to bind sub report if we put sub report in main report ?

here is my problem :

code snippet :

DataTable dtExtraDoc = dtDocSub.Select("DocumentId=14").Length == 0 ? null : dtDocSub.Select("DocumentId=14").CopyToDataTable();
if (dtExtraDoc != null && dtExtraDoc.Rows.Count > 0)
{
  if (dtExtraDoc.Rows[0]["ResponceId"].ToString() == "2")
  {
    repdoc.Subreports["subRpt-extra-Doc-textile-claim-query-letter.rpt"].SetDataSource(dtExtraDoc);
  }
}

If dtExtraDoc is null then it will not bind the sub report and hence it gives me error " Object reference not set to an instance of object"

I am totally new to crystal reports please help me..

Thanks in advance.