Undefined array key 1 in Drupal\sendgrid_integration\Plugin\Mail\SendGridMail->doMail()

Problem/Motivation

If the parseAddress() returns array with only e-mail, than there is no array element with key 1. That cause the warning.
in SendGridMail.php:

Proposed resolution

Check if $fromaddrarray[1] isset, otherwise set site name:
$data['fromname'] = isset($fromaddrarray[1]) ? strval($fromaddrarray[1]) : strval($sitename);