How to handle null exception when exporting grid with foreign key columns in .NET WebForms application?

@(Html.EJ().Grid<object>("FlatGrid")
        .Datasource((IEnumerable<object>)ViewBag.dataSource) 
        .AllowPaging()
        .ToolbarSettings(toolbar =>
        {
            toolbar.ShowToolbar(true)
            .ToolbarItems(
                items =>
                {                    
                    items.AddTool(ToolBarItems.ExcelExport);
                    items.AddTool(ToolBarItems.WordExport);
                    items.AddTool(ToolBarItems.PdfExport);
                });
        }) 
        .Mappers(map => map.ExportToExcelAction("/Home/ExportToExcel").ExportToPdfAction("/Home/ExportToPdf").ExportToWordAction("/Home/ExportToWord"))
        .Columns(col =>
        {
            col.Field("OrderID").HeaderText("Order ID").IsPrimaryKey(true).Width(90).Add();
            col.Field("CustomerID").HeaderText("Customer ID").Width(90).Add();
            col.Field("EmployeeID").HeaderText("Employee Name").ForeignKeyField("EmployeeID").ForeignKeyValue("FirstName").DataSource((IEnumerable<object>)ViewBag.data).Width(75).Add();
            col.Field("Freight").HeaderText("Freight").Width(75).Format("{0:C}").Add();
            col.Field("ShipCity").HeaderText("Ship City").Width(80).Add();
        })
)

public class HomeController : Controller
    {
        
        public ActionResult Index()
        {
            ViewBag.dataSource = OrderRepository.GetAllRecords().ToList();  
            ViewBag.data = EmployeeRepository.GetAllRecords().ToList();
            return View();
        }        

        public void ExportToExcel(string GridModel)
        {
            ExcelExport exp = new ExcelExport();
            
            var dataSource = OrderRepository.GetAllRecords().ToList();
 
            GridProperties obj = ConvertGridObject(GridModel);
            obj.Columns[2].DataSource = EmployeeRepository.GetAllRecords().ToList();//set the dataSource for the foreign key column after deserializing the grid model properties.
            exp.Export(obj, dataSource, "Pedidos.xlsx", Syncfusion.XlsIO.ExcelVersion.Excel2010, false, false, "flat-saffron");
        }
        
        public void ExportToWord(string GridModel)
        {
            WordExport exp = new WordExport();
            var dataSource = OrderRepository.GetAllRecords().ToList();
            GridProperties obj = ConvertGridObject(GridModel);
            obj.Columns[2].DataSource = EmployeeRepository.GetAllRecords().ToList();//set the dataSource for the foreign key column after deserializing the grid model properties.
            exp.Export(obj, dataSource, "Pedidos.docx", false, false, "flat-saffron");
        }
        
        public void ExportToPdf(string GridModel)
        {
            PdfExport exp = new PdfExport();
            var dataSource = OrderRepository.GetAllRecords().ToList();
            GridProperties obj = ConvertGridObject(GridModel);
            obj.Columns[2].DataSource = EmployeeRepository.GetAllRecords().ToList();//set the dataSource for the foreign key column after deserializing the grid model properties.
            exp.Export(obj, dataSource, "Pedidos.pdf", false, false, true, "flat-saffron");
        }
        
        private GridProperties ConvertGridObject(string gridProperty)
        {
            JavaScriptSerializer serializer = new JavaScriptSerializer();
            IEnumerable div = (IEnumerable)serializer.Deserialize(gridProperty, typeof(IEnumerable));
            GridProperties gridProp = new GridProperties();
            foreach (KeyValuePair<string, object> ds in div)
            {
                var property = gridProp.GetType().GetProperty(ds.Key, BindingFlags.Instance | BindingFlags.Public | BindingFlags.IgnoreCase);
                if (property != null)
                {
                    Type type = property.PropertyType;
                    string serialize = serializer.Serialize(ds.Value);
                    object value = serializer.Deserialize(serialize, type);
                    property.SetValue(gridProp, value, null);
                }
            }
            return gridProp;
        }