You're not alone. Installed public beta 3 this morning & exactly the same here. Force touching , replying from notification centre etc is exactly the same. Just a white blank space with no messages & no keyboard to reply to anything. There's just a x in top right corner to clear. Also it then goes a bit wild & starts jumping all over. I turned phone off once & back on & cleared , but soon came back. I've read deleting the message thread & starting a new one helps , but that didn't for me. Maybe a reboot , but i don't believe that's a permanent cure. I have a 6S+
[doublepost=1470232468][/doublepost]Ok so update.
I rebooted & it worked via force touch from lock screen 3 times. The 4th message i received i opened phone , pulled down NC & force touched message from there. Fail ! Didn't work & blank again & nothing.