Hi @mailjet
I have the same error on deactivating plugin.
PHP Fatal error: ?Uncaught TypeError: call_user_func_array(): Argument #1 ($callback) must be a valid callback, class "MailjetPlugin\Includes\MailjetDeactivator" not found in ...\wp-includes\class-wp-hook.php:308
It seems to be a problem with your composer autoload.
