Unsupported operand types error when creating new Webform handler
When I look at line 263 in \Drupal\webform\Plugin\WebformHandlerBase there is the below code...

$this-&gt;configuration = $configuration['settings'] + $this-&gt;defaultConfiguration();


Your Unsupported operand types error message indicates that one of those variables/properties is not an array.

My best guess is that \Drupal\webform\Plugin\WebformHandlerBase::defaultConfiguration is not returning an array() because your $configuration['settings'] is set to an empty array.