generic_builder.dart

import 'package:built_collection/built_collection.dart';
import 'package:flutter/material.dart';
import 'package:flutter_form_builder/flutter_form_builder.dart';

// generic widget definition

typedef Widget FormWidgetBuilder<T>(T t, Color textColor);

class FormBuilderListSelect<T> extends StatefulWidget {

  .
  .
  .
  
  final FormWidgetBuilder<T> widgetBuilder;
  
  .
  .
  .
}

// then, call it like this:

 @override
  Widget build(BuildContext context) {

      return FormBuilderListSelect<YourGenericType>(widgetBuilder: (item, color) => Padding(...));
  }