Virtual keyboard not popping up on 'Messages' (chat).
 

This has happened to me many times. When I swipe away from the chat window and come back I cannot write anymore because vkb doesn't show up. Tried with different chat protocols and sms without success. If I want to reply I have to restart my device.
What process should be killed/restarted to be able to write again? I haven't modded the keyboard.