Always define app.source, don't restore nonexistent source

satellite/application.py

@@ -65,6 +65,8 @@ class SatelliteApp(Gtk.Application):
             self.app_menu)
         self.menu_popover.set_position(Gtk.PositionType.BOTTOM)
 
+        self.source = None
+
         self.infolabel.set_markup("<tt>" + "\n"*10 + "</tt>")
 
         self.dataframe = DataFrame()
@@ -186,7 +188,8 @@ class SatelliteApp(Gtk.Application):
         """Called after main loop exits."""
         print("Cleaning up...")
         self.gpx_write()
-        self.source.restore()
+        if self.source is not None:
+            self.source.restore()
         print("...done.")
 
     def sigint_handler(self):