onChange:(v){
_textController = v;  ///* this line create an issue if remove then works fine
}

import 'package:flutter/material.dart';

void main() {
  runApp(MaterialApp(
    home: MyForm(),
  ));
}

class MyForm extends StatefulWidget {
  @override
  _MyFormState createState() => _MyFormState();
}

class _MyFormState extends State<MyForm> {
  final _formKey = GlobalKey<FormState>();
  final _textController = TextEditingController();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('TextFormField Example'),
      ),
      body: Form(
        key: _formKey,
        child: Padding(
          padding: const EdgeInsets.all(16.0),
          child: Column(
            crossAxisAlignment: CrossAxisAlignment.start,
            children: [
              TextFormField(
                controller: _textController,
                decoration: InputDecoration(
                  labelText: 'Enter your text',
                ),
onChange:(v){
_textController = v;  ///* this line create an issue if remove then works fine
}
                validator: (value) {
                  if (value == null || value.isEmpty) {
                    return 'Please enter some text';
                  }
                  return null;
                },
              ),
              SizedBox(height: 16.0),
              ElevatedButton(
                onPressed: () {
                  if (_formKey.currentState!.validate()) {
                    // Form is valid, proceed with your logic
                    String enteredText = _textController.text;
                    print('Entered text: $enteredText');
                  }
                },
                child: Text('Submit'),
              ),
            ],
          ),
        ),
      ),
    );
  }

  @override
  void dispose() {
    _textController.dispose();
    super.dispose();
  }
}