Getting this error message in Flutter The argument type 'String? Function(String)' can't be assigned to the parameter type 'String?)?'.

For the line listed below - 

if (inValue.length == 0)

How do I resolve it?

How to resolve this error message in flutter - library_private_types_in_public_api

The line of code it's complaining is  this -

_MyPageState createState() => _MyPageState();



The flutter code lines - class MyPage extends StatefulWidget {
const MyPage({super.key});

@override
_MyPageState createState() => _MyPageState();
}

Write a program called Lab 5 that has 3 methods and a main method:
1) Method isUserNameValid has 1 parameter of type string and the method returns a value of type
boolean. If the parameter has less than 8 characters then the method returns false, otherwise it
returns true.
2) Method compare has 3 parameters all of type int and has a String return type. This method will
return
a. “All Same” if all 3 parameters are equal,
b. “All Different” if all 3 parameters are different.
c. “Mix number(s)” two of the numbers are the same and the 3rd is different.
3) Method isValid has 1 parameter of type int and returns a boolean value; the method returns true if
the parameter is in the range 0,…,100 inclusive, otherwise the method returns false.
4) Main method. I want you to test methods isValid & isUsernameValid one time with a valid data
and another time with an invalid data. As for method compare test it 3 times so you can get the 3
different possible outcomes.
Example to test the isUserNameValid method

18. What is the value of the String referred to by the variable s after the following code executes? String fun = "bcd"; String a = ""; for (int i = 0; i < fun.length(); i++) { a = a + (char) (fun.charAt(i) + 3); String s = ""; for (int i = 0; i < a.length(); i++) { S = S + (char) (a.charAt(i) + 'A' -'a');