validate method
A function that returns an error message string
to display if the input s
not pass a validation
test and returns null otherwise.
Implementation
@override
String? validate(String input) {
int l = input.length;
if (l < min) {
return "Please enter a value that is at least $min characters long.";
}
if (l > max) {
return "Please enter a value that does not exceed $max characters in length.";
}
return null;
}