Impossible to create a `dup` backup action on Debian using ninjahelper

On Debian-based systems (I tested on Debian 11, "Bullseye", and Ubuntu 18.04, "Bionic"), it seems impossible to use ninjahelper to create a dup backup action, as trying to select "src" directories results in the error:

/usr/share/backupninja/dup.helper: line 484: do_dup_src: command not found

Version of backupninja used

1.2.1-1

(Installed from the Debian 11 repository.)

Steps to reproduce

This can be reproduced by running Debian in a Docker container -- docker run --rm -it debian:bullseye.

Within the container:

1. install duplicity and backupninja:

$ apt-get update && apt-get install -y --no-install-recommends duplicity backupninja

2. Run ninjahelper:

$ ninjahelper

3. Select "new / create a new backup action"

4. Select "dup / incremental encrypted remote filesystem backup"

5. Select "src / choose files to include and exclude"

Expected behaviour

The user should be able to specify files to include and exclude.

Actual behaviour

The error message

/usr/share/backupninja/dup.helper: line 484: do_dup_src: command not found

briefly appears at the bottom of the screen, and it's impossible to progress further through the ninjahelper "wizard".