MVC Linq throws intermittent null reference exception during export to Excel - asp.net-mvc-5

Question

I'm using the same code in 6 Areas, each a separate database. Been working fine for a long time, now fails in 3 of the 6 with a null reference exception. Classes all the same, all database tables hold data. First method is the view which displays the data, second called by a link in the view to export the data to Excel. 3 cases display and export correctly. In 2 out of the other 3 cases the data is displayed correctly in the view, and the null exception is thrown at the point the data should be being retrieved in the export method. In the last case the null exception is thrown when trying to return the view. Simply can't understand how it works in some instances but not in others.
public ActionResult PVS(string studySite, string currentFilter, int? page)
{
ViewBag.SelectedID = studySite;
if (studySite != null)
page = 1;
else
studySite = currentFilter;
ViewBag.CurrentFilter = studySite;
IQueryable<PVS> schedule = db.PVS.OrderBy(v => v.SiteNumber).ThenBy(v => v.Participant);
if (studySite != null)
{
string selectedID = studySite;
images = schedule.Where(v => v.SiteNumber == selectedID);
}
int pageSize = 10;
int pageNumber = (page ?? 1);
return View(schedule.ToPagedList(pageNumber, pageSize));
}
public void ExportPVSToExcel()
{
var grid = new System.Web.UI.WebControls.GridView();
var pvs = db.PVS.OrderBy(p => p.SiteNumber).ThenBy(p => p.Participant).ToList();
grid.DataSource = pvs;
grid.DataBind();
// create an Excel worksheet for the data export
ExcelPackage excel = new ExcelPackage();
var workSheet = excel.Workbook.Worksheets.Add("PVS");
var totalCols = grid.Rows[0].Cells.Count;
var totalRows = grid.Rows.Count;
var headerRow = grid.HeaderRow;
for (var i = 1; i <= totalCols; i++)
{
workSheet.Cells[1, i].Value = headerRow.Cells[i - 1].Text;
}
for (var j = 1; j <= totalRows; j++)
{
for (var i = 1; i <= totalCols; i++)
{
var data = pvs.ElementAt(j - 1);
workSheet.Cells[j + 1, i].Value = data.GetType().GetProperty(headerRow.Cells[i - 1].Text).GetValue(data, null);
}
}
using (var memoryStream = new MemoryStream())
{
Response.ContentType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";
Response.AddHeader("content-disposition", "attachment; filename=PVS.xlsx");
excel.SaveAs(memoryStream);
memoryStream.WriteTo(Response.OutputStream);
Response.Flush();
Response.End();
}
}