TypeError: "in_array(): Argument #2 ($haystack) must be of type array for workflows which do not apply to nodes

When having a workflow which does not apply to a node type the module throws a PHP Exception:

TypeError: "in_array(): Argument #2 ($haystack) must be of type array, null given" at content_notification/src/ContentNotificationService.php line 159

This is because $settings['entity_types']['node'] is not set in this case.

To reproduce add a new workflow which does not apply to any node type and open an existing content-notification configuration or create a new one and check the ajax requests when changing the content type.