Error: Call to a member function __toString() on string

Problem/Motivation

In RouteInModalAjaxHelper::ajaxCallback(), the following

...
$messages = \Drupal::messenger()->all();
$config = \Drupal::config('route_in_modal.settings');

if (!isset($messages['error'])) {
...
if ($confirmationMessagesModal && count($messages) > 0) {
        \Drupal::messenger()->deleteAll();

        $display = strip_tags($messages['status'][0]->__toString());

throws

Error: Call to a member function __toString() on string

if a message is a string and not a MarkupInterface.

In addition, only the first message is shown while all are deleted.
Also, there is no guarantee that the first message will be the one that is desired to be shown.
As far as I'm aware, there is no way to determine from a message where came from and if it is directly related to the action just performed, whatever it may be..

Proposed resolution

I think the safest approach for now is to show all messages, which also might include notices.