kivy: save and open to last wallet

The previous code did not work as before the flow reaches the old save point window.wallet is set to None in on_stop.

gui/kivy/__init__.py

@@ -60,5 +60,3 @@ class ElectrumGui:
                            plugins = self.plugins,
                            gui_object=self)
         w.run()
-        if w.wallet:
-            self.config.save_last_wallet(w.wallet)

gui/kivy/main_window.py

@@ -511,6 +511,8 @@ 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):