Renabled verification & refactored window closing.

core/src/toga/app.py

@@ -554,6 +554,15 @@ def _startup(self):
         # This is a wrapper around the user's startup method that performs any
         # post-setup validation.
         self.startup()
+        self._verify_startup()
+
+    def _verify_startup(self):
+        if self.main_window is not None:
+            if not isinstance(self.main_window, MainWindow):
+                raise ValueError(
+                    "toga.App.main_window must be of type MainWindow. "
+                    "Does your startup() method assign a value of type MainWindow to self.main_window?"
+                )
 
     def about(self) -> None:
         """Display the About dialog for the app.

winforms/src/toga_winforms/window.py

@@ -134,31 +134,17 @@ def winforms_FormClosing(self, sender, event):
         # If the app is exiting, or a manual close has been requested,
         # don't get confirmation; just close.
         if not self.interface.app._impl._is_exiting and not self._is_closing:
-            # For handling window closing depending on its closeability
-            # settings or when there is an on_close event handler.
-            if not self.interface.closeable or self.interface.on_close._raw:
-                # If there is an on_close event handler, process it;
-                # but then cancel the close event. If the result of
-                # on_close handling indicates the window should close,
-                # then it will be manually triggered as part of that
-                # result handling.
-                self.interface.on_close(self)
-                # Close the app when the main_window or last window closes.
-                if (self.interface == self.interface.app.main_window) or (
-                    len(self.interface.app.windows) == 0 and self.interface.app.on_exit
-                ):
-                    self.interface.app.on_exit(self.interface.app)
-
-            # For handling window closing when no on_close handler is provided.
-            elif self.interface.on_close._raw is None:
-                self.interface.close()
-                # Close the app when the main_window or last window closes.
-                if (self.interface == self.interface.app.main_window) or (
-                    len(self.interface.app.windows) == 0 and self.interface.app.on_exit
-                ):
-                    self.interface.app.on_exit(self.interface.app)
-
             # Closeability is implemented by shortcutting the close handler.
+            if self.interface.closeable:
+                if self.interface.on_close._raw is not None:
+                    # If there is an on_close event handler, process it;
+                    # but then cancel the close event. If the result of
+                    # on_close handling indicates the window should close,
+                    # then it will be manually triggered as part of that
+                    # result handling.
+                    self.interface.on_close(self)
+                elif self.interface.on_close._raw is None:
+                    self.interface.close()
             event.Cancel = True
 
     def set_full_screen(self, is_full_screen):