Add specific validator when calling widget

I’m trying to call this widget which contains FormField with it’s validator as a variable.

    import 'package:flutter/material.dart';

class InputField extends StatelessWidget {
  final String label, label2;
  final String content, content2;
  var fieldController = TextEditingController();
  void fieldValidator =() {
    
  };

  InputField(
      {this.label,
      this.content,
      this.label2,
      this.content2,
      this.fieldValidator
      });

  @override
  Widget build(BuildContext context) {
    return LayoutBuilder(
      builder: (BuildContext context, BoxConstraints constraints) {
        return Row(
          children: <Widget>[
            Expanded(
              flex: 1,
              child: Container(
                width: 50.0,
                child: Text(
                  "$label",
                  textAlign: TextAlign.left,
                  style: TextStyle(
                    fontWeight: FontWeight.w900,
                    color: Color.fromARGB(255, 255, 255, 255),
                  ),
                ),
              ),
            ),
            SizedBox(
              width: 5.0,
            ),
            Expanded(
              flex: 3,
              child: Container(
                width: MediaQuery.of(context).size.width / 3.7,
                color: Color.fromARGB(255, 255, 255, 255),
                child: TextFormField(
                  controller: fieldController,
                  validator: fieldValidator,
                  style: TextStyle(
                    fontSize: 15.0,
                    color: Colors.black,
                  ),
                  decoration: InputDecoration(
                    contentPadding: EdgeInsets.all(10.0),
                    hintText: "$content",
                    hintStyle: TextStyle(
                        color: Color.fromARGB(255, 190, 190, 190),
                        fontSize: 14),
                    fillColor: Color.fromARGB(255, 0, 0, 0),
                  ),
                ),
              ),
            ),
          ],
        );
      },
    );
  }
}

I’m setting the validator in the widget constractor , and I want to give my own validator function every time I call the widget like this :

InputField( label: "City",
           content: "Please provide us your city name",
         fieldValidator:(value) {
                            if (value == null || value.isEmpty) {
                              return 'This one is required';
                            }
                            return null;
                          },),

I’m getting this error on the first page where I created my widget:

    This expression has a type of 'void' so its value can't be used.
Try checking to see if you're using the correct API; there might be a function or call that returns void you didn't expect. Also check type parameters and variables which might also be void.

>Solution :

the validator should be from the type of FormFieldValidator<String>? instead of void so your validator should look like:

FormFieldValidator<String>? fieldValidator =(fieldValue) { 
  return "result";
  };