From a user interface perspective I don't agree with hiding/disabling the paper plane icon as this can cause users to either be confused about the purpose of the grey rounded rect at the bottom of their screen, or be confused about why their action to tap the paper plane icon does nothing.
I know we all know what these symbols mean but that icon can be a critical affordance to non-technical users.
Ideally a message would appear letting the user know they should type a message into the text field at the bottom of their screen.
