Magento 2 Notice: Trying to access array offset on value of type null in

Notice: Trying to access array offset on value of type null in /home/u1230712/*******.com.tr/vendor/magento/module-captcha/Model/DefaultModel.php on line 485

When i entered to the admin panel, this error is showing. How can i fix the problem? I added the error code below.

line 485: public function getWord()
{
$sessionData = $this->session->getData($this->getFormIdKey(self::SESSION_WORD));
return time() < $sessionData[‘expires’] ? $sessionData[‘data’] : null;
}

How i can fix the problem?