This very much looks like a bug int Aptdaemon's PK compatibility layer to me. At this point, it likely makes no sense to fix this issue, since that code has been dropped after the Xenial release.
@tlc: This will likely be a won't fix bug, please upgrade to Bionic if you can. If you can't upgrade, you can try to switch from the compat layer to the real PK daemon, if the system allows you to do that:
sudo apt purge python3-aptdaemon.pkcompat ; sudo apt install packagekit
This very much looks like a bug int Aptdaemon's PK compatibility layer to me. At this point, it likely makes no sense to fix this issue, since that code has been dropped after the Xenial release.
@tlc: This will likely be a won't fix bug, please upgrade to Bionic if you can. If you can't upgrade, you can try to switch from the compat layer to the real PK daemon, if the system allows you to do that: aptdaemon. pkcompat ; sudo apt install packagekit
sudo apt purge python3-