System.Argument Null Exception Value cannot be null.Parameter name: value

if (CaseStatusDay >= maxDays)
{
  if (currentrow != null)
  {
    currentrow.BeginEdit();
    if (showFlagCaseStatus.Contains(currentCaseStatus))
    {
      currentrow["CaseStatusFlag"] = "~/Images/flag_red.png";

      if (webSession.GetVariable("LoginCompanyType") == "CCCS")
      {
        if (e.Row.Cells.Count >= 13)
        {
          e.Row.Cells[12].Attributes.Add("onmouseover", "tooltip('Case has not been statused in the last " + maxdaysdispay + " days')");
          e.Row.Cells[12].Attributes.Add("onmouseout", "exit();");
        }
      }
      else if (webSession.GetVariable("LoginCompanyType") == "VEND" || webSession.GetVariable("LoginCompanyType") == "INS")
      {
        if (e.Row.Cells.Count >= 14)
        {
          e.Row.Cells[13].Attributes.Add("onmouseover", "tooltip('Case has not been statused in the last " + maxdaysdispay + " days')");
          e.Row.Cells[13].Attributes.Add("onmouseout", "exit();");
        }
      }
    }
  }
}