Otherwise you may call shutdown on a non-existant attribute.
If you start up the app and close it without making a wallet, you will see this error currently:
Traceback (most recent call last):
File "tinywallet/app.py", line 155, in shutdown
if self.assetScreen is not None:
AttributeError: 'TinyWallet' object has no attribute 'assetScreen'
Otherwise you may call shutdown on a non-existant attribute.
If you start up the app and close it without making a wallet, you will see this error currently: