[FIX] XLSXImport: Fix crash on incomplete xlsx file with external reference

In Excel, there is a mecanism for spreadsheets to cross-reference
themselves, with cells of the form `[otherSpreadsheetFileName]Sheet1!A1`.
Luckily, the saved files contain a cache of the values from other
spreadsheets and we use this cache during our import process to replace
the cross-sheet references with their cached value.

Google sheet, and possibly other spreadsheets engines, do not support
xlsx files with cross references to other spreadsheets. More specifically,
they let them *as is* even though the reference will not work, and they
throw the cache away. This means that an xlsx file generated from Google
Sheet will not contain the cache either.

The code was designed with the assumption that the cache would always be
present, which led to crashes when trying to upload an xlsx file (with
cross-references) that was obtain through Google Sheets.

closes #7879

Task: 5499753
X-original-commit: 2ed5b7c
Signed-off-by: Lucas Lefèvre (lul) <[email protected]>

Author: rrahir

src/xlsx/conversion/formula_conversion.ts

@@ -65,10 +65,10 @@ function convertFormula(formula: string, data: XLSXImportData): string {
   formula = formula.replace(externalReferenceRegex, (match, externalRefId, sheetName, cellRef) => {
     externalRefId = Number(externalRefId) - 1;
     cellRef = cellRef.replace(/\$/g, "");
-
-    const sheetIndex = data.externalBooks[externalRefId].sheetNames.findIndex((name) =>
-      isSheetNameEqual(name, sheetName)
-    );
+    const sheetIndex =
+      data.externalBooks[externalRefId]?.sheetNames.findIndex((name) =>
+        isSheetNameEqual(name, sheetName)
+      ) ?? -1;
     if (sheetIndex === -1) {
       return match;
     }