Steps to Reproduce

1. load the attached xlsx
Or
Test case io.keikai.model.FormulaEvalTest.keikai457()

Current Result

2021-03-29 14:35:26 [ERROR] FormulaEngineImpl:491 - null when eval IF(B1>10, 1,0)
java.lang.NullPointerException
at org.zkoss.poi.ss.formula.eval.RelationalOperationEval.evaluate(RelationalOperationEval.java:72)
at org.zkoss.poi.ss.formula.functions.Fixed2ArgFunction.evaluate(Fixed2ArgFunction.java:48)
at org.zkoss.poi.ss.formula.OperationEvaluatorFactory.evaluate(OperationEvaluatorFactory.java:134)
at org.zkoss.poi.ss.formula.WorkbookEvaluator.evaluateFormula(WorkbookEvaluator.java:625)
at org.zkoss.poi.ss.formula.WorkbookEvaluator.evaluateAny(WorkbookEvaluator.java:355)
at org.zkoss.poi.ss.formula.WorkbookEvaluator.evaluate(WorkbookEvaluator.java:294)
at io.keikai.model.impl.sys.formula.FormulaEngineImpl.evaluateFormula(FormulaEngineImpl.java:587)
at io.keikai.model.impl.sys.formula.FormulaEngineImpl.evaluate(FormulaEngineImpl.java:467)
at io.keikai.model.impl.CellImpl.evalFormula(CellImpl.java:201)
at io.keikai.model.impl.CellImpl.getFormulaResultType(CellImpl.java:218)
at io.keikai.ui.SpreadsheetCtrlImpl.getCellAttr(SpreadsheetCtrlImpl.java:495)
at io.keikai.ui.SpreadsheetCtrlImpl.getRangeAttrs(SpreadsheetCtrlImpl.java:382)
at io.keikai.ui.Spreadsheet.createActiveRange(Spreadsheet.java:1970)
at io.keikai.ui.Spreadsheet.renderProperties(Spreadsheet.java:1923)

Expected Result

no error

Debug Information

1. in RelationalOperationEval, context becomes null after calculating vA with OperandResolver.getSingleValue()

			vA = context.isEvalNameFormula()
					? OperandResolver.getSingleValueInNameFormula(arg0, srcRowIndex, srcColumnIndex)
					: OperandResolver.getSingleValue(arg0, srcRowIndex, srcColumnIndex);
			vB = context.isEvalNameFormula()
					? OperandResolver.getSingleValueInNameFormula(arg1, srcRowIndex, srcColumnIndex)
					: OperandResolver.getSingleValue(arg1, srcRowIndex, srcColumnIndex);

RelationalOperationEval is used as singleton, so while evaluating one RelationalOperationEval and evaluating another RelationalOperationEval will turn context into null. Should not put context in a singleton

Workaround