From 916b6888ebd5bf0ebbfdcc775b4089096d0bdc6a Mon Sep 17 00:00:00 2001
From: Jan Sverre Riksfjord <jasverix@gmail.com>
Date: Thu, 10 Dec 2020 22:52:00 +0100
Subject: [PATCH] worksheet: fix if cellValue does not exist (#1727)

The condition is FALSE if the cell does not exist in the flipped table,
but anyway, it is sent in to a method requiring 'string' type, causing
it to fail.
---
 src/PhpSpreadsheet/Writer/Xlsx/Worksheet.php | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/PhpSpreadsheet/Writer/Xlsx/Worksheet.php b/src/PhpSpreadsheet/Writer/Xlsx/Worksheet.php
index b6a6fc39..8faa7ae2 100644
--- a/src/PhpSpreadsheet/Writer/Xlsx/Worksheet.php
+++ b/src/PhpSpreadsheet/Writer/Xlsx/Worksheet.php
@@ -1079,7 +1079,7 @@ class Worksheet extends WriterPart
     {
         $objWriter->writeAttribute('t', $mappedType);
         if (!$cellValue instanceof RichText) {
-            self::writeElementIf($objWriter, isset($pFlippedStringTable[$cellValue]), 'v', $pFlippedStringTable[$cellValue]);
+            self::writeElementIf($objWriter, isset($pFlippedStringTable[$cellValue]), 'v', $pFlippedStringTable[$cellValue] ?? '');
         } else {
             $objWriter->writeElement('v', $pFlippedStringTable[$cellValue->getHashCode()]);
         }