I found a fix:
in file mailinabox/setup/questions.sh, on line 95 you will see:
if [[ -z "$DEFAULT_PUBLIC_IP" && ! -z "$GUESSED_IP" ]]; then
Change this line to:
if [[ -z "${DEFAULT_PUBLIC_IP:-}" && ! -z "$GUESSED_IP" ]]; then
This is what solved the problem for me, as of 13 December 2018.