The data validation rule should have values for formula1 and formula2 elements, which define the start and end values for a range to validate between. When such values are missing, a NullReferenceException is thrown on import and the document cannot be imported.
Details:
System.NullReferenceException: Object reference not set to an instance of an object.
at Telerik.Windows.Documents.Spreadsheet.FormatProviders.OpenXml.Xlsx.Model.Elements.Worksheets.DataValidationElement.CopyPropertiesToSingleArgumentContext(SingleArgumentDataValidationRuleContext context)
at Telerik.Windows.Documents.Spreadsheet.FormatProviders.OpenXml.Xlsx.Model.Elements.Worksheets.DataValidationElement.CopyPropertiesToDataValidationRuleContext(DataValidationRuleContextBase context)
at Telerik.Windows.Documents.Spreadsheet.FormatProviders.OpenXml.Xlsx.Model.Elements.Worksheets.DataValidationElement.ListRuleFactory(IXlsxWorksheetImportContext context)
at Telerik.Windows.Documents.Spreadsheet.FormatProviders.OpenXml.Xlsx.Model.Elements.Worksheets.DataValidationElement.OnAfterRead(IXlsxWorksheetImportContext context)
...
at Telerik.Windows.Documents.Spreadsheet.FormatProviders.WorkbookFormatProviderBase.Import(Stream input)