Failed to show preferences dialogue

[enmus]

Actually, this is the solution

Panel is gone

@Qubes_User171 should start terminal in dom0, execute xfce4-panel. Panel will show up. Then it is needed forcibly to close the terminal regardless of the warning. The panel will disappear and reappear in a flash second. It usually happens when using context menu while on panel which somehow triggers closing the panel without the warning.