I can reproduce the bug in 16.04 but it seems to be fixed in 18.04:
$ echo '' | bluetoothctl No agent is registered $
$ bluetoothctl < /dev/null [bluetooth]# quitt to bluetoothd... No agent is registered $
I'm going to declare this fix released and nominate 17.10 for a fix (whatever the fix would be).
I can reproduce the bug in 16.04 but it seems to be fixed in 18.04:
$ echo '' | bluetoothctl
No agent is registered
$
$ bluetoothctl < /dev/null
[bluetooth]# quitt to bluetoothd...
No agent is registered
$
I'm going to declare this fix released and nominate 17.10 for a fix (whatever the fix would be).