why the custom widget onchange event did not triggered – Flutter

I have defined a custom widget in flutter(version 3.7.11) like this:

// ignore_for_file: prefer_typing_uninitialized_variables

import 'package:flutter/material.dart';
import 'package:google_fonts/google_fonts.dart';
import 'package:hexcolor/hexcolor.dart';

class MyTextField extends StatelessWidget {
  final controller;
  final String hintText;
  final bool obscureText;
  final Icon prefixIcon;
  final Function()? onChanged;

  const MyTextField(
      {super.key,
      required this.controller,
      required this.hintText,
      required this.obscureText,
      required this.prefixIcon,
      this.onChanged});

  @override
  Widget build(BuildContext context) {
    return TextField(
      controller: controller,
      obscureText: obscureText,
      cursorColor: HexColor("#4f4f4f"),
      decoration: InputDecoration(
        hintText: hintText,
        fillColor: HexColor("#f0f3f1"),
        contentPadding: const EdgeInsets.fromLTRB(20, 20, 20, 20),
        hintStyle: GoogleFonts.poppins(
          fontSize: 15,
          color: HexColor("#8d8d8d"),
        ),
        border: OutlineInputBorder(
          borderRadius: BorderRadius.circular(30),
          borderSide: BorderSide.none,
        ),
        prefixIcon: prefixIcon,
        prefixIconColor: HexColor("#4f4f4f"),
        filled: true,
      ),
    );
  }
}

and I am using this widget like this:

import 'package:flutter/material.dart';
import 'package:untitled4/my_textfield.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    void validatePhone() {
      print("object");
    }

    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: Scaffold(body:
      SafeArea(
          child:MyTextField(
              controller: null,
              onChanged: ()=>validatePhone(),
              hintText: 'dddd',
            obscureText: false,
            prefixIcon: const Icon(Icons.phone_android_outlined)
            )
      )),
    );
  }
}

now I found the MyTextField event did not trigger onchange event. why did this happen? Am I missing something? what should I do to fixed this issue? this is the dependencies I am using:

  hexcolor: ^3.0.1
  google_fonts: ^3.0.1