Change 'Window' object has no attribute 'viewport' error to something more useful

[uwuclxdy]

What is the problem or limitation you are having?

When trying to add a new window to the app the error 'Window' object has no attribute 'viewport' appeared and I had no idea what it meant. Turns out I needed to add the window to self.app.windows before window.show()

e.g.:

def open_interface(self, widget):
    main_box = toga.Box()
    new_window = toga.Window(title="ttttt")
    new_window.content = main_box
    new_window.show()

A call to self.app.windows += new_window is needed before new_window is shown,

Describe the solution you'd like

A more clear error message, indicating on what I did wrong. A window can detect if it's been assigned to an app; if a window hasn't been assigned to an app, it should raise an appropriate error, rather than raising errors as a side effect when deeper problems occur.

Also - if you're on iOS or Android, you can't create additional windows at all (because it wouldn't make sense to do so); attempting to add a window should raise an error.

Describe alternatives you've considered

Asking for help in the official BeeWare discord server.

Additional context

No response