Problem/Motivation
At time to test I got:
Behat\Testwork\Call\Exception\CallErrorException: 8192: strpos(): Passing null to parameter #1 ($haystack) of type string is deprecated in /var/www/html/web/core/lib/Drupal/Core/Mail/Plugin/Mail/PhpMail.php
in PhpMail.php 80 and 112 lines can be the reasons.
Steps to reproduce
run
$ drush status-report
Proposed resolution
Valid the values befor to call to strpos function