Importing an XLSX workbook can crash with System.ArgumentNullException: Value cannot be null. Parameter name: key during Workbook.ResumeCalculationMapRegistration().

Root cause:
• The XLSX import pipeline queued null RadExpression entries into pendingCalculationMapExpressions.
• CalculationMap.AddBatch() passed those entries into ExpressionDependencyMap.AddExpressionsBatch().
• GetOrCreateNodeWithCache() then called Dictionary.TryGetValue() with a null key, which threw the exception.