PHP 8.1 compatibility

Author: PowerKiKi

src/PhpSpreadsheet/Calculation/MathTrig/Subtotal.php

@@ -39,7 +39,10 @@ function ($index) use ($cellReference) {
                 if ($cellReference->getWorksheet()->cellExists($column . $row)) {
                     //take this cell out if it contains the SUBTOTAL or AGGREGATE functions in a formula
                     $isFormula = $cellReference->getWorksheet()->getCell($column . $row)->isFormula();
-                    $cellFormula = !preg_match('/^=.*\b(SUBTOTAL|AGGREGATE)\s*\(/i', $cellReference->getWorksheet()->getCell($column . $row)->getValue());
+                    $cellFormula = !preg_match(
+                        '/^=.*\b(SUBTOTAL|AGGREGATE)\s*\(/i',
+                        $cellReference->getWorksheet()->getCell($column . $row)->getValue() ?? ''
+                    );
 
                     $retVal = !$isFormula || $cellFormula;
                 }