PostEvent not pointing to any valid message

In beliefset, PostEvent property.

We previously can add a PostEvent (by pressing the '+' image button ) and then have it not yet have any valid messages, and have multiple invalid records and save it.

Current Solutions : * We disable the addition '+' button if we have already 1 invalid slot of record of PostEvent to be filled. * When the widget property is out of scope, we delete any invalid record.