Text message forwarding (null) code?

My IPad has suddenly stopped receiving my text messages from my iPhone SE, when I try to re-install the text message forwarding I get the (null) code message on my iPad. I have tried turning both Iphone and iPad off, turned Apple ID off and logged back in and still get the same code which you obviously can't use! Anybody know how to fix it? Had numerous issues with iPhone SE in the past.