Disabling a button

I'm trying to make this Done text view be disabled if the user has not entered anything in the text field
Code Block
@Binding var activityName: String
TextField("Enter activity name", text: $activityName)
.navigationBarItems(trailing: Text("Done")
              .disabled(self.activityName.isEmpty))

Does anyone know a fix to this?
What do you get ? Is button disabled at strt when empty ?
What does not work, enabling when you enter something ?

Could you show the complete code of ContentView ?
Disabling a button
 
 
Q