This error is show when there is no "Object to pass" configured in the Show Page action. So I would suggest looking at the usages of the MessageMainOverview and MyAccount page and double check that the configuration of the Show Page activity is correct.
The page needs as main object the ChatUser object. So if you want to start a chat you have to connect your user to a ChatUser. Most of the time you can resolve this to let the ChatUser be a generalization of your user object. This way each user of your application is automaticly a ChatUser.
When opening the form you need to pass the ChatUser object. If you use generalization you can just pass the currentuser object to that form.