Export to excel fails when there is null values for GridViewDateTimeColumn.
The following code snippet reproduce the issue:

ExportToExcelML export = new ExportToExcelML(gridView);
export.HiddenColumnOption = HiddenOption.DoNotExport;
export.HiddenRowOption = HiddenOption.DoNotExport;
export.ExportVisualSettings = false;
export.SheetMaxRows = ExcelMaxRows._1048576;
export.RunExport(targetFileName);