Why is the code provided in flutter europe not working for me

In a flutter talk made in 2020, rendering and elements were mentioned. I was writing the same codes to learn, but there were errors. What's wrong with the code?

link = https://www.youtube.com/watch?v=PnWxW21vDak&t=564s

import 'package:flutter/cupertino.dart';
import 'package:flutter/rendering.dart';

class MyCenter extends SingleChildRenderObjectWidget{
  MyCenter({Key? key,Widget? child}) : super(key: key,child: child);

  

  @override
  RenderObject createRenderObject(BuildContext context) {
    return _RenderMyCenter();
  }
}


class _RenderMyCenter extends RenderBox with RenderObjectWithChildMixin<RenderBox>{


  @override
  void performLayout(){
    
    if(child != null){
      child!.layout(constraints.loosen(),parentUsesSize: true);
      double dx = (constraints.maxWidth - child!.size.width)/2;
      double dy = (constraints.maxHeight - child!.size.height)/2;

      BoxParentData childParentData = child!.parentData as BoxParentData;
      childParentData.offset = Offset(dx,dy);

    }else{
      size = const Size(0,0);
    }

    @override
    void paint(PaintingContext context,Offset offset){
      if(child!= null){
        var parentData = child!.parentData as BoxParentData;
        context.paintChild(child as RenderObject, offset+parentData.offset);
      }
    }
  
  }

}  

Then I tried to run the above class but it gave an error.

error = [ERROR:flutter/lib/ui/ui_dart_state.cc(198)] Unhandled Exception: Cannot hit test a render box that has never been laid out. The hitTest() method was called on this RenderBox: RenderStack#8503f NEEDS-LAYOUT NEEDS-PAINT: creator: Stack ← _FloatingActionButtonTransition ← MediaQuery ← LayoutId-[<_ScaffoldSlot.floatingActionButton>] ← CustomMultiChildLayout ← AnimatedBuilder ← DefaultTextStyle ← AnimatedDefaultTextStyle ← _InkFeatures-[GlobalKey#f49e3 ink renderer] ← NotificationListener ← PhysicalModel ← AnimatedPhysicalModel ← ⋯ parentData: offset=Offset(0.0, 0.0); id=_ScaffoldSlot.floatingActionButton

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:flutter_detay/WidgetRenderElement/render1.dart';

void main(){

  runApp(testApp());

}

class testApp extends StatelessWidget {
  const testApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: testHome(),
    );
  }
}

class testHome extends StatefulWidget {
  testHome({Key? key}) : super(key: key);

  @override
  State<testHome> createState() => _testHomeState();
}

class _testHomeState extends State<testHome> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(

      ),
      body: MyCenter(child: Text("HEYYOO"),),
    );
  }
}