Smartfarm registration validation [Application]

<Widget>[
                            Container(
                              child: TextFormField(
                                controller: username,
                                decoration:
                                    InputDecoration(labelText: 'Username'),
                                validator: (value) {
                                  if (value.isEmpty) {
                                    return 'Username cannot be empty';
                                  }
                                },
                              ),
                            ),
                            Container(
                              margin: const EdgeInsets.only(top: 12),
                              child: TextFormField(
                                controller: password,
                                obscureText: true,
                                decoration:
                                    InputDecoration(labelText: 'Password'),
                                validator: (value) {
                                  if (value.isEmpty) {
                                    return 'Password cannot be empty';
                                  }
                                },
                              ),
                            ),
                            Container(
                              margin: const EdgeInsets.only(top: 12),
                              child: TextFormField(
                                controller: email,
                                decoration: InputDecoration(labelText: 'Email'),
                                validator: (value) {
                                  RegExp emailValidator = new RegExp(
                                      r'^(([^<>()\[\]\\.,;:\s@"]+(\.[^<>()\[\]\\.,;:\s@"]+)*)|(".+"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$');
                                  if (!emailValidator.hasMatch(value)) {
                                    return 'Email is invalid';
                                  }
                                },
                              ),
                            ),
                            Container(
                              margin: const EdgeInsets.only(top: 12),
                              child: TextFormField(
                                  controller: address,
                                  decoration:
                                      InputDecoration(labelText: 'Address'),
                                  validator: (value) {
                                    if (value.isEmpty) {
                                      return 'Address cannot be empty';
                                    }
                                  }),
                            ),
                            Container(
                              margin: const EdgeInsets.only(top: 12),
                              child: TextFormField(
                                  controller: nationalID,
                                  decoration:
                                      InputDecoration(labelText: 'National ID'),
                                  keyboardType: TextInputType.number,
                                  validator: validateNationalID),
                            ),
                            Container(
                              margin: const EdgeInsets.only(top: 12),
                              child: new DropdownButton<String>(
                                items: constants.AvailableProvince.map(
                                    (String value) {
                                  return new DropdownMenuItem<String>(
                                    value: value,
                                    child: new Text(value),
                                  );
                                }).toList(),
                                onChanged: (value) {
                                  setState(() {
                                    province = value;
                                  });
                                },
                                hint: Text("Province"),
                                value: province,
                                isExpanded: true,
                              ),
                            ),
                          ]