xfce4-panel: Cannot open display: . Type "xfce4-panel --help" for usage.

Usage: xfce4-panel [OPTION?] [ARGUMENTS...] Help Options: -h, --help Show help options --help-all Show all help options --help-gtk Show GTK+ Options --help-sm-client Show session management options Application Options: -p, --preferences=PANEL-NUMBER Show the 'Panel Preferences' dialog -a, --add-items=PANEL-NUMBER Show the 'Add New Items' dialog -s, --save Save the panel configuration --add=PLUGIN-NAME Add a new plugin to the panel -r, --restart Restart the running panel instance -q, --quit Quit the running panel instance -d, --disable-wm-check Do not wait for a window manager on startup -V, --version Print version information and exit --display=DISPLAY X display to use