Flutter TextEditingController clear wont reset the error message

We can use InputDecoration to toggle the errorStyle and achieve the desired result.

Output:

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final appTitle = 'Form Validation Demo';

    return MaterialApp(
      title: appTitle,
      home: Scaffold(
        appBar: AppBar(
          title: Text(appTitle),
        ),
        body: MyCustomForm(),
      ),
    );
  }
}

// Create a Form widget.
class MyCustomForm extends StatefulWidget {
  @override
  MyCustomFormState createState() {
    return MyCustomFormState();
  }
}

// Create a corresponding State class.
// This class holds data related to the form.
class MyCustomFormState extends State<MyCustomForm> {
  // Create a global key that uniquely identifies the Form widget
  // and allows validation of the form.
  //
  // Note: This is a GlobalKey<FormState>,
  // not a GlobalKey<MyCustomFormState>.
  final _formKey = GlobalKey<FormState>();
  final TextEditingController taskController = TextEditingController();
  bool hideError = false;
  bool first = true;

  @override
  Widget build(BuildContext context) {
    // Build a Form widget using the _formKey created above.
    return Form(
      key: _formKey,
      autovalidateMode: AutovalidateMode.onUserInteraction,
      child: Column(
        crossAxisAlignment: CrossAxisAlignment.start,
        children: <Widget>[
          TextFormField(
              controller: taskController,
              decoration: hideError && !first
                  ? InputDecoration(
                      errorStyle: TextStyle(height: 0),
                      )
                  : null,
              validator: (value) {
                if (value.isEmpty) {
                  print(value);

                  return 'Please enter some text';
                }
                return null;
              },
              onChanged: (value) {
                setState(() {
                  hideError = false;
                  first = false;
                });
              }),
          Padding(
            padding: const EdgeInsets.symmetric(vertical: 16.0),
            child: ElevatedButton(
              onPressed: () {
                // Validate returns true if the form is valid, or false
                // otherwise.
                setState(() {
                  hideError = true;
                });
                if (_formKey.currentState.validate()) {
                  // If the form is valid, display a Snackbar and reset
                  
                  Scaffold.of(context)
                      .showSnackBar(SnackBar(content: Text('Processing Data')));
                  taskController.clear();
                  
                }
              },
              child: Text('Submit'),
            ),
          ),
        ],
      ),
    );
  }
}

If you do not want to show error message then simple delete the validator.

validator: (value) {
          if (value.isEmpty) {
            return 'Please enter some text';
          }
          return null;
        },

remove this from your code !!

validator: (String value) {
          if (value.isEmpty) {
            return ' ';
          }
          return null;
        },

Do this, by returning an empty String you can still see the text Field red.