Trying to fix a flutter form widget from a widget book written in 2019. I fixed a few
but having issues with some parts of the code. String? Function(String?) validator = (String? inValue)
Flutter says too many positional arguments, but 1 found.
Try removing the extra positional arguments or specifying the name for named arguments
Abstract classes can't be instantiated.
Try creating an instance of a concrete subsytem
Add required position parameter.

Here's more of the code TextFormField(
keyboardType:
TextInputType.emailAddress,
String? Function(String?) validator = (String? inValue) {
if (inValue.length == 0) {
//do something
return "Please enter username";
}
return null;

Hi so I did this to try to resolve The argument type 'String? Function(String)' can't be assigned to the parameter type 'String?)?'

But it still complains about that error message I thought the //do something will fix the issue ,but it still didn't work. Flutter code below

if (inValue.length == 0) {
//do something
return "Please enter username";
}
return null;
},

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?

42. The bake method of the CookieJar class should take as input arguments the type of cookie to be baked, the number of cookies to be baked, and what the calories of each of these cookies will be (calPerCookie). No return value is needed. The bake method should be accessible from other classes. What is the correct method header? (A) public void bake (String type, int number, int calPerCookie) (B) private static void bake (String type, int number, int calperCookie) (C) public static void bake (String type, int number, int calperCookie) (D) private void bake (String type, int number, int calPerCookie) (E) public int bake (String type, int number, int calperCookie)