MDL-45818 core_message: use system context if user no longer exists

message/lib.php

@@ -2375,11 +2375,18 @@ function message_mark_message_read($message, $timeread, $messageworkingempty=fal
 
     $DB->delete_records('message', array('id' => $messageid));
 
+    // Get the context for the user who received the message.
+    $context = context_user::instance($message->useridto, IGNORE_MISSING);
+    // If the user no longer exists the context value will be false, in this case use the system context.
+    if ($context === false) {
+        $context = context_system::instance();
+    }
+
     // Trigger event for reading a message.
     $event = \core\event\message_viewed::create(array(
         'objectid' => $messagereadid,
         'userid' => $message->useridto, // Using the user who read the message as they are the ones performing the action.
-        'context'  => context_user::instance($message->useridto),
+        'context' => $context,
         'relateduserid' => $message->useridfrom,
         'other' => array(
             'messageid' => $messageid