`validate()` is always counting on `is_valid` property of `$ret` which is a `stdClass` object. I encountered two scenarios where this error: ``` PHP Notice: Undefined property: stdClass::$is_valid in vendor/overint/php-mailgun-validation/src/MailgunValidator.php on line 61 ``` is being thrown: - I had an invalid `$apiKey` and then I tried doing `$mailgunValidator->validate('some@email.com')` - I had a valid `$apiKey` and then I tried doing `$mailgunValidator->validate('')`