Steps to Reproduce

1. import the file with zk.xml

    <library-property>
        <name>io.keikai.import.cache</name>
        <value>true</value>
    </library-property>

Current Result

java.lang.IllegalArgumentException: value must not be null
org.zkoss.poi.ss.formula.eval.StringEval.<init>(StringEval.java:40)
io.keikai.importer.XlsxImporter.importCell(XlsxImporter.java:1370)
io.keikai.importer.XlsxImporter.importRow(XlsxImporter.java:1278)
io.keikai.importer.XlsxImporter.imports(XlsxImporter.java:249)
io.keikai.range.impl.imexp.ExcelImportAdapter.imports(ExcelImportAdapter.java:48)
io.keikai.range.impl.imexp.AbstractImporter.imports(AbstractImporter.java:57)
io.keikai.ui.Spreadsheet.getSBook(Spreadsheet.java:672)
io.keikai.ui.Spreadsheet.getSelectedSSheet(Spreadsheet.java:871)
io.keikai.ui.Spreadsheet.doInvalidate(Spreadsheet.java:5545)
io.keikai.ui.Spreadsheet.invalidate(Spreadsheet.java:4392)
io.keikai.ui.Spreadsheet.setSrc(Spreadsheet.java:911)

Expected Result

successfully imported without the error

Debug Information

• if I turn off the cache, keikai imports the file successfully.