**Symfony version(s) affected**: master **Description** <!-- A clear and concise description of the problem. --> The variable `fileLinkFormat` is initialized everywhere in the class `Symfony\Component\Debug\ExceptionHandler` to null but here it is asking for a string ``` public function setFileLinkFormat($fileLinkFormat): string { $old = $this->fileLinkFormat; $this->fileLinkFormat = $fileLinkFormat; return $old; } ``` **How to reproduce** <!-- Code and/or config needed to reproduce the problem. If it's a complex bug, create a "bug reproducer" as explained in: https://symfony.com/doc/current/contributing/code/reproducer.html --> - `git clone git@github.com:symfony/website-skeleton.git .` - `composer install` **Additional context** <!-- Optional: any other context about the problem: log messages, screenshots, etc. --> The return type was added in PR #31996