only happened to me once using beta 5 and happend once in beta 6. i go to open the message app and immediately when i select an existing thread, the app force closes. if i go back and close it from the multitasking menu, it still doesnt work. the only way to get back into the app and open a thread is to reboot. i am however able to start a new message or access an existing thread by going to their contact and select send message.