Fix two deprecated warnings:
PHP Deprecated: Creation of dynamic property XLSXWriter::$tempdir is deprecated in vendor/mk-j/php_xlsxwriter/xlsxwriter.class.php on line 44
PHP Deprecated: htmlspecialchars(): Passing null to parameter #1 ($string) of type string is deprecated in vendor/mk-j/php_xlsxwriter/xlsxwriter.class.php on line 760