Erro no then do save contact

Ao clicar no botão para criar o novo contato, ocorre o seguinte erro:

The method 'then' was called on null.
Receiver: null
Tried calling: then<bool>(Closure: (int) => bool)

Não executa o pop do navigator, voltando para lista o contato foi salvo

código do botão

 RaisedButton(
                  child: Text('Create'),
                  onPressed: () {
                    final String name = _nameController.text;
                    final int accountNumber =
                        int.tryParse(_accountNumberController.text);

                    final Contact newContact = Contact(0, name, accountNumber);
                    save(newContact).then((id) => Navigator.pop(context));
                  },
                ),

Código do save

Future<int> save(Contact contact) {
  createDatabase().then((db) {
    final Map<String, dynamic> contactMap = Map();
    contactMap['name'] = contact.name;
    contactMap['account_number'] = contact.accountNumber;

    return db.insert('contacts', contactMap);
  });
}

Código completo do form

import 'package:flutter/material.dart';
import 'package:gringotts/database/app_database.dart';
import 'package:gringotts/models/contact.dart';

class ContactForm extends StatefulWidget {
  @override
  _ContactFormState createState() => _ContactFormState();
}

class _ContactFormState extends State<ContactForm> {
  final TextEditingController _nameController = TextEditingController();

  final TextEditingController _accountNumberController =
      TextEditingController();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('New Contact'),
      ),
      body: Padding(
        padding: const EdgeInsets.all(16.0),
        child: Column(
          children: <Widget>[
            Padding(
              padding: const EdgeInsets.only(top: 8.0),
              child: TextField(
                  controller: _nameController,
                  decoration: InputDecoration(
                    labelText: 'Full Name',
                  ),
                  style: TextStyle(
                    fontSize: 24.0,
                  )),
            ),
            Padding(
              padding: const EdgeInsets.only(top: 8.0),
              child: TextField(
                controller: _accountNumberController,
                decoration: InputDecoration(
                  labelText: 'Account number',
                ),
                style: TextStyle(
                  fontSize: 24.0,
                ),
                keyboardType: TextInputType.number,
              ),
            ),
            Padding(
              padding: const EdgeInsets.only(top: 16.0),
              child: SizedBox(
                width: double.maxFinite,
                child: RaisedButton(
                  child: Text('Create'),
                  onPressed: () {
                    final String name = _nameController.text;
                    final int accountNumber =
                        int.tryParse(_accountNumberController.text);

                    final Contact newContact = Contact(0, name, accountNumber);
                    save(newContact).then<bool>((id) => Navigator.pop(context));
                  },
                ),
              ),
            ),
          ],
        ),
      ),
    );
  }
}