Lesson 11 - Register.dart and signin.dart are creating issues with the toggle view.

The code ` final Function toggleView;
SignIn({this.toggleView});`
and
` final Function toggleView;
Register({this.toggleView});`

are both returning this error:

**The parameter 'toggleView' can't have a value of 'null' because of its type, but the implicit default value is 'null'. (Documentation) Try adding either an explicit non-'null' default value or the 'required' modifier.**