If a cell is formatted with the custom format "h:mm:ss", reading the Text property of a Cell will cause a System.NullReferenceException exception to be thrown. This is being caused by the following line in ExcelNumberFormatXml.AddBuildIn:
NumberFormats.Add("h:mm:dd", new ExcelNumberFormatXml(NameSpaceManager, true) { NumFmtId = 21, Format = "h:mm:dd" });
The format should be h:mm:ss, not h:mm:dd.
NumberFormats.Add("h:mm:dd", new ExcelNumberFormatXml(NameSpaceManager, true) { NumFmtId = 21, Format = "h:mm:dd" });
The format should be h:mm:ss, not h:mm:dd.