kivy: more reliable saving of "last wallet"

The intended behaviour is that when the user launches the app,
the wallet we try to open is the wallet most recently opened by the user.
The old code in 'on_stop' in practice only got called if the user exited via
repeatedly pressing the system 'back' button.

related #5952

electrum/gui/kivy/main_window.py

@@ -658,8 +658,6 @@ class ElectrumWindow(App):
 
     def on_stop(self):
         Logger.info('on_stop')
-        if self.wallet:
-            self.electrum_config.save_last_wallet(self.wallet)
         self.stop_wallet()
 
     def stop_wallet(self):
@@ -833,6 +831,7 @@ class ElectrumWindow(App):
             send_exception_to_crash_reporter(e)
             return
         self.use_change = self.wallet.use_change
+        self.electrum_config.save_last_wallet(wallet)
 
     def update_status(self, *dt):
         if not self.wallet: