我正在进行验证过程，但我想在不同的页面之间进行验证。如何使用formKey或其他解决方案在另一个页面上执行验证过程？我得到了一个空错误，如何摆脱它？
我的主文件在下面

import 'package:flutter/material.dart';
import 'package:intl_flutter_deneme/buton.dart';
import 'package:intl_phone_number_input/intl_phone_number_input.dart';
import 'package:mask_text_input_formatter/mask_text_input_formatter.dart';
import 'package:share_plus/share_plus.dart';

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

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

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(primarySwatch: Colors.amber),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({super.key});

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  TextEditingController phoneController = TextEditingController();
  bool isButtonActive = false;
  final _formKey = GlobalKey<FormState>();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('phone number settings'),
      ),
      body: Center(
        child: Column(
          children: [
            Form(
              key: _formKey,
              child: InternationalPhoneNumberInput(
                countries: ['TR'],
                onInputChanged: (PhoneNumber number) {
                  print(number.phoneNumber);
                },
                maxLength: 13,
                hintText: '5## ### ## ##',
                validator: (deger) {
                  if(deger!.isEmpty && deger.length < 13) {
                    return 'doğru şekilde numara gir';
                  }return null;
                },
                spaceBetweenSelectorAndTextField: 0,
                inputDecoration: InputDecoration(),
                textFieldController: phoneController,
              ),
            ),
            ButonIslemi(),
                
            ElevatedButton(
                onPressed: sharedMessage, child: Text('share')),
          ],
        ),
      ),
    );
  }

  void sharedMessage() {
    String message = 'https://www.instagram.com/atakansever_/';
    Share.share(message);
  }
}

按下按钮时执行验证的部分如下所示

import 'package:flutter/material.dart';
import 'package:intl_flutter_deneme/main.dart';

class ButonIslemi extends StatelessWidget {
  ButonIslemi({super.key});
  final formKey = GlobalKey<FormState>();
  @override
  Widget build(BuildContext context) {
    return ElevatedButton(onPressed: () {
      if(formKey.currentState!.validate()) {
        print('işlem başarılı');
      }
    }, child: Text('bas'));
  }
}

我按下按钮时得到的错误如下