Fixed winforms implementation.

Author: proneon267

winforms/src/toga_winforms/window.py

@@ -178,9 +178,11 @@ def resize_content(self):
         )
 
     def window_on_gain_focus(self, sender, event):
-        self.interface.app.on_gain_focus(self.interface)
+        if self.interface.app is not None:
+            self.interface.app.on_gain_focus(self.interface)
         self.interface.on_gain_focus(self.interface)
 
     def window_on_lose_focus(self, sender, event):
-        self.interface.app.on_lose_focus(self.interface)
+        if self.interface.app is not None:
+            self.interface.app.on_lose_focus(self.interface)
         self.interface.on_lose_focus(self.interface)