I've just hit the issue myself.
The value of the field was indeed ''.
I set the config with drush -y config-set symfony_mailer_lite.message text_format plain_text and now it works as expected.
We had migrated from Swift mailer straight onto 1.0.6.
However I couldn't replicate with a fresh install of the module on another site.
Add a new argument so you can set and pass it.
{# Add new argument #}
{% macro menu_links(items, attributes, menu_level, parentItem) %}
{# Set the new argument to what you need, e.g. when the parent link is output #}
{% set parentItem = item %}
{% if menu_level == 1 %}
{# Check if the parent menu item exists #}
{% if parentItem %}
{# Display the parent menu item's title in the submenu #}
<li class="menu-item--submenu-parent">
<a href="{{ parentItem.url }}" class="submenu-parent-link">{{ parentItem.title }}</a>
</li>
{% endif %}
Hopefully that helps.
Just in case someone is using laragon on Windows you can use something similar to the below:
$config['symfony_mailer.mailer_transport.sendmail']['configuration']['query']['command'] = ini_get('sendmail_path') . ' -t';
$settings['mailer_sendmail_commands'] = [
ini_get('sendmail_path') . ' -t'
];
Works for me too, be good to get a release out :)