I test ubuntu_19.04-xfce-4.14pre2 docker container in xfce-test in VirtualBox.
Power manager plugin does not start.
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