[PropertyAccess] Fix access to undefined "file" key when checking stack frames

Author: nicolas-grekas

src/Symfony/Component/PropertyAccess/PropertyAccessor.php

@@ -470,7 +470,7 @@ private function readProperty(array $zval, string $property, bool $ignoreInvalid
                         [$trace] = $e->getTrace();
 
                         // handle uninitialized properties in PHP >= 7
-                        if (__FILE__ === $trace['file']
+                        if (__FILE__ === ($trace['file'] ?? null)
                             && $name === $trace['function']
                             && $object instanceof $trace['class']
                             && preg_match('/Return value (?:of .*::\w+\(\) )?must be of (?:the )?type (\w+), null returned$/', $e->getMessage(), $matches)