3 errors in make components in flutter

I have 3 errors
I tried all possible solutions
look at this

Widget defaultFormField ({
  @required TextEditingController? controller,
  @required TextInputType? keyboardType,
  @required IconData? prefix,
  @required String? label,
  VoidCallback? onChange,
  @required VoidCallback? validate,

}) => TextFormField(
    style: const TextStyle(
      color: Colors.amber,
    ),
    cursorHeight: 25.0,
    cursorColor: Colors.amber,
    controller: controller,
    keyboardType: keyboardType,
    decoration: const InputDecoration(
      prefixIcon: Icon(Icons.person_outline),
      labelText: label,
      labelStyle: TextStyle(
        color: Colors.amber,
      ),
      border: OutlineInputBorder(),
      enabledBorder: OutlineInputBorder(
        borderSide: BorderSide(
            color: Colors.amber
        ),
      ),
      focusedBorder: OutlineInputBorder(
        borderSide: BorderSide(
          color: Colors.amber,
        ),
      ),
    ),
    onChanged: onChange,
    validator : validate,
);

these are problems

Invalid constant value.

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The argument type ‘void Function()?’ can’t be assigned to the parameter type ‘void Function(String)?’.

The argument type ‘void Function()?’ can’t be assigned to the parameter type ‘void Function(String)?’.

>Solution :

1. remove const from this line decoration: const InputDecoration( .

2. required Function? onChange,

3. required Function? validator,

use onChanged like this

 onChanged: (value) {
         return onChanged != null ? onChanged(value) : null;
  },

use validator like this

validator: (value) {
            return validator != null ? validator(value) : null;
          },

try above code it can solve your error.
and remove @ from required.