No way Sexton could have rolled away in that instance and if Barnes wasn't 100% sure, he could have just given penalty and warning. He could have also giving a decent advantage to Wales and see what came from that - but no. Straight to the card. Why not even try and advantage when you are that close to the line.
Wales gave away 3 penalties in strong attacking positions and didn't give one. This for me was the turning point of the match and sorry - thought it wasn't the right call.
You can't be 100% certain in all decisions.