I'm on Xubuntu 19.04, fully patched with xfpanel-switch version 1.0.7-ubuntu2 and I'm having the same or similar problem as noted by the original reporter.  When I start xfpanel-switch manually, here are the errors reported upon startup, when the first save configuration is pressed, and finally when Save Configuration is selected from the pop-up menu.  This may have existed in 18.10 (I don't know) but I upgraded to 19.04 recently.  The last saved config is labeled Xubuntu Disco from 08-21-19.  I cannot remember if I made this manually or if it was done automatically as part of the upgrade.

234->  xfpanel-switch
/usr/share/xfpanel-switch/xfpanel-switch/xfpanel-switch.py:132: DeprecationWarning: Gtk.Widget.override_background_color is deprecated
  self.xfce_header.override_background_color(0, base_color[1])
/usr/share/xfpanel-switch/xfpanel-switch/xfpanel-switch.py:134: DeprecationWarning: Gtk.Widget.override_color is deprecated
  self.xfce_header.override_color(0, fg_color[1])


[press save configuration button]

/usr/share/xfpanel-switch/xfpanel-switch/xfpanel-switch.py:309: PyGTKDeprecationWarning: The keyword(s) "message_format" have been deprecated in favor of "text" respectively. See: https://wiki.gnome.org/PyGObject/InitializerDeprecations
  _("Save Configuration"), Gtk.ResponseType.ACCEPT))
/usr/share/xfpanel-switch/xfpanel-switch/xfpanel-switch.py:309: PyGTKDeprecationWarning: The "buttons" argument must be a Gtk.ButtonsType enum value. Please use the "add_buttons" method for adding buttons. See: https://wiki.gnome.org/PyGObject/InitializerDeprecations
  _("Save Configuration"), Gtk.ResponseType.ACCEPT))


[press Save Configuration to make backup from pop-up menu]

Traceback (most recent call last):
  File "/usr/share/xfpanel-switch/xfpanel-switch/xfpanel-switch.py", line 221, in on_save_clicked
    self.save_configuration(name)
  File "/usr/share/xfpanel-switch/xfpanel-switch/xfpanel-switch.py", line 209, in save_configuration
    PanelConfig.from_xfconf(self.xfconf).to_file(filename)
  File "/usr/share/xfpanel-switch/xfpanel-switch/panelconfig.py", line 65, in from_xfconf
    pc.find_desktops()
  File "/usr/share/xfpanel-switch/xfpanel-switch/panelconfig.py", line 148, in find_desktops
    self.remove_keys(rem_keys)
  File "/usr/share/xfpanel-switch/xfpanel-switch/panelconfig.py", line 155, in remove_keys
    del self.properties[param]
KeyError: '/plugins/plugin-48'


Nothing happens and nothing is saved.  An attempt to export the configuration fails as follows:

[press the export button]

/usr/share/xfpanel-switch/xfpanel-switch/xfpanel-switch.py:99: PyGTKDeprecationWarning: Using positional arguments with the GObject constructor has been deprecated. Please specify keyword(s) for "title, parent, action, buttons" or use a class specific constructor. See: https://wiki.gnome.org/PyGObject/InitializerDeprecations
  button, Gtk.ResponseType.ACCEPT))
/usr/share/xfpanel-switch/xfpanel-switch/xfpanel-switch.py:99: PyGTKDeprecationWarning: The "buttons" argument must be a Gtk.ButtonsType enum value. Please use the "add_buttons" method for adding buttons. See: https://wiki.gnome.org/PyGObject/InitializerDeprecations
  button, Gtk.ResponseType.ACCEPT))

[Export configuration as ... pop-up appears.  Regardless of where you navigate, pressing Save fails]

Traceback (most recent call last):
  File "/usr/share/xfpanel-switch/xfpanel-switch/xfpanel-switch.py", line 234, in on_export_clicked
    self.save_configuration(filename, False)
  File "/usr/share/xfpanel-switch/xfpanel-switch/xfpanel-switch.py", line 209, in save_configuration
    PanelConfig.from_xfconf(self.xfconf).to_file(filename)
  File "/usr/share/xfpanel-switch/xfpanel-switch/panelconfig.py", line 65, in from_xfconf
    pc.find_desktops()
  File "/usr/share/xfpanel-switch/xfpanel-switch/panelconfig.py", line 148, in find_desktops
    self.remove_keys(rem_keys)
  File "/usr/share/xfpanel-switch/xfpanel-switch/panelconfig.py", line 155, in remove_keys
    del self.properties[param]
KeyError: '/plugins/plugin-48'

[Nothing is written]


I have not deleted panels, but I have struggled with Xfce4 on my dual-headed system since support for independent parallel desktops running (non-Xinerama mode) on each screen in Xubuntu 18.10 was broken.  This may be due to moving to gtk3, but it is a serious degradation of service since xfdesktop and xfce4-panel no longer honor the display option.  (Note: xfwm4 still works on two independent displays!)  I mention this first and foremost because I want to see this fixed, and second because there were two different panels defined on this system at one point -- one for each display -- and that may have some bearing on what may be causing this problem.

If you need further config info from this system, please let be know.  I'm not currently sure where to look for the panel definitions and don't want to much up my current working panel.