So you've got your answer. The aia project you added has the "default device" theme set. Change the theme back to "Dark" and it should be ok. It works for me.
You need to change the logic of the "AfterTextInput" handler to be independent of the "cancel" button. As you can see, this button depends on the language set on the phone. My language says "Anuluj" and your blocking logic is not working.
The only strange thing is that when you switch back to the default theme, the text color of the notification buttons doesn't change to black. There are still white inscriptions and you can't see them. I think this is a bug in AppInventor.
Plus, these white frames have a nice effect and a modern look.
The only strange thing is that when you switch back to the default theme, the text color of the notification buttons doesn't change to black. There are still white inscriptions and you can't see them. I think this is a bug in App Inventor.
I tested it again in a test app. However, this is not a bug, just a strange color logic . The colors of the notification buttons are specified in Screen1 with the "AccentColor" property. In the app above it is set to white, so you cannot see the subtitles in the default theme.