how to show error message when the user click the sign up button without filling the forms in flutter code example

Example 1: create a validator in flutter

TextFormField(
  // The validator receives the text that the user has entered.
  validator: (value) {
    if (value.isEmpty) {
      return 'Please enter some text';
    }
    return null;
  },
);

Example 2: create a validator in flutter

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

// Define 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>();

  @override
  Widget build(BuildContext context) {
    // Build a Form widget using the _formKey created above.
    return Form(
      key: _formKey,
      child: Column(
        children: <Widget>[
              // Add TextFormFields and ElevatedButton here.
        ]
     )
    );
  }
}

Example 3: create a validator in flutter

ElevatedButton(
  onPressed: () {
    // Validate returns true if the form is valid, otherwise false.
    if (_formKey.currentState.validate()) {
      // If the form is valid, display a snackbar. In the real world,
      // you'd often call a server or save the information in a database.

      Scaffold
          .of(context)
          .showSnackBar(SnackBar(content: Text('Processing Data')));
    }
  },
  child: Text('Submit'),
);