Dropdown validation

Try using DropdownButtonFormField wrapping inside Form

Ex:

import 'package:flutter/material.dart';

class FormValidationWithDropdown extends StatefulWidget {
  @override
  _FormValidationWithDropdownState createState() =>
      _FormValidationWithDropdownState();
}

class _FormValidationWithDropdownState
    extends State<FormValidationWithDropdown> {
  final _formKey = GlobalKey<FormState>();
  bool _autovalidate = false;
  String selectedSalutation;
  String name;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Form(
        key: _formKey,
        autovalidate: _autovalidate,
        child: Column(
          children: <Widget>[
            DropdownButtonFormField<String>(
              value: selectedSalutation,
              hint: Text(
                'Salutation',
              ),
              onChanged: (salutation) =>
                  setState(() => selectedSalutation = salutation),
              validator: (value) => value == null ? 'field required' : null,
              items:
                  ['MR.', 'MS.'].map<DropdownMenuItem<String>>((String value) {
                return DropdownMenuItem<String>(
                  value: value,
                  child: Text(value),
                );
              }).toList(),
            ),
            TextFormField(
              decoration: InputDecoration(hintText: 'Name'),
              validator: (value) => value.isEmpty ? 'Name is required' : null,
              onSaved: (value) => name = value,
            ),
            FlatButton(
              child: Text('PROCEED'),
              color: Colors.green,
              onPressed: () {
                if (_formKey.currentState.validate()) {
                  //form is valid, proceed further
                  _formKey.currentState.save();//save once fields are valid, onSaved method invoked for every form fields

                } else {
                  setState(() {
                    _autovalidate = true; //enable realtime validation
                  });
                }
              },
            )
          ],
        ),
      ),
    );
  }
}

How to validate dropdown button in flutter?Stack Overflow