I test ubuntu_19.04-xfce-4.14pre2 docker container in xfce-test in VirtualBox. Power manager plugin does not start. Dialog says: Plugin "Power Manager Plugin" unexpectedly left the panel, do you want to restart it? The plugin restarted more then once in the last 60 seconds... I dont see the plugin in https://bugzilla.xfce.org/enter_bug.cgi?classification=Panel%20Plugins
The plugin doesn't work in the container due to a lack of upower in docker, so this is not a bug but a limitation of docker.
For reference see https://github.com/schuellerf/xfce-test/commit/83702d7d9cd698db224539e56afcc6e7a3ae5d38