Hi, I have experienced the same error.
After updating to the newest version of the plugin (3.1.1) the problem still occurs when uploading to the media library a .docx file or .pdf generated from the .docx file.
Error message below:
Error Message: Uncaught TypeError: Argument 1 passed to WebpConverter\Conversion\Media\Upload::init_attachment_convert() must be of the type array, null given, called in /public_html/wp-includes/class-wp-hook.php on line 292 and defined in /public_html/wp-content/plugins/webp-converter-for-media/src/Conversion/Media/Upload.php:39