MessageNotifierBase: restricts rendered fields to user/cck fields only

[OnkelTem]

I extended Message entity with a base field which I use to store the output.
Then I pass it in 'rendered fields' array of the notifier options.
And it doesn't work in \Drupal\message_notify\Plugin\Notifier\MessageNotifierBase::postSend because it expects the field must be a cck-one which is not the case in my situation.
Particularly this check breaks everything:

if (!$field = $this->entityTypeManager->getStorage('field_config')->load('message.' . $this->message->bundle() . '.' . $field_name)) {
    throw new MessageNotifyException('Field "' . $field_name . '"" does not exist.');
}

I propose replacing this with the basic: $this->message->hasField($field_name) and not bothering with deducing the nature of the field provided.

[OnkelTem]

This is against 8.x-1.0-beta1:

diff --git a/src/Plugin/Notifier/MessageNotifierBase.php b/src/Plugin/Notifier/MessageNotifierBase.php
index 7b2682a..bb68917 100644
--- a/src/Plugin/Notifier/MessageNotifierBase.php
+++ b/src/Plugin/Notifier/MessageNotifierBase.php
@@ -137,8 +137,7 @@ abstract class MessageNotifierBase extends PluginBase implements MessageNotifier
         }
         $field_name = $this->configuration['rendered fields'][$view_mode];
 
-        // @todo Inject the content_type.manager if this check is needed.
-        if (!$field = $this->entityTypeManager->getStorage('field_config')->load('message.' . $this->message->bundle() . '.' . $field_name)) {
+        if (!$this->message->hasField($field_name)) {
           throw new MessageNotifyException('Field "' . $field_name . '"" does not exist.');
         }