Steps to reproduce
Just send message using the queue system
Here is the error log when the message is sent:
Notice: Trying to access array offset on value of type bool in Drupal\message_subscribe\Subscribers->sendMessage() (line 255 of modules/contrib/message_subscribe/src/Subscribers.php).
Drupal\message_subscribe\Subscribers->sendMessage(Object, Object, Array, Array, Array) (Line: 69)
Drupal\message_subscribe\Plugin\QueueWorker\MessageSubscribe->processItem(Array) (Line: 109)
Drupal\ultimate_cron\QueueWorker->queueCallback(Object)
Proposed resolution
In modules/contrib/message_subscribe/src/Subscribers.php, line 255, change $subscribe_options['queue']['end time'] into $subscribe_options['end time'] as it looks like this 'end time option is configured separately; see line 148 in the same file:
$subscribe_options += [
'save message' => TRUE,
'skip context' => FALSE,
'last uid' => 0,
'uids' => [],
'range' => $use_queue ? 100 : FALSE,
'end time' => FALSE,
'use queue' => $use_queue,
'queue' => FALSE,
'entity access' => TRUE,
'notify blocked users' => FALSE,
'notify message owner' => $notify_message_owner,
];
Remaining tasks
Create a patch