FIX: removes destroyed mentioned used (#21841)

This would generate a 500 as we would call id on a nil object.

plugins/chat/app/serializers/chat/message_serializer.rb

@@ -30,6 +30,7 @@ module Chat
       object
         .chat_mentions
         .map(&:user)
+        .compact
         .sort_by(&:id)
         .map { |user| BasicUserWithStatusSerializer.new(user, root: false) }
         .as_json