qt init: more clean-up for system tray

related: https://github.com/spesmilo/electrum/issues/6889#issuecomment-812776695

electrum/gui/qt/__init__.py

@@ -392,6 +392,8 @@ class ElectrumGui(Logger):
             self.app.sendEvent(self.app.clipboard(), event)
             if self.tray:
                 self.tray.hide()
+                self.tray.deleteLater()
+                self.tray = None
         self.app.aboutToQuit.connect(clean_up)
 
         # main loop

electrum/gui/qt/main_window.py

@@ -3184,6 +3184,8 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, Logger):
         if self._update_check_thread:
             self._update_check_thread.exit()
             self._update_check_thread.wait()
+        if self.tray:
+            self.tray = None
         self.gui_object.timer.timeout.disconnect(self.timer_actions)
         self.gui_object.close_window(self)