DropdownWidget.dart

Padding(
                  padding: const EdgeInsets.only(top: 20.0, right: 45.0, left: 45.0),
                    child: new FormField(
                    builder: (FormFieldState state) {
                      return InputDecorator(
                        decoration: InputDecoration(
                          icon: const Icon(Icons.business),
                          labelText: 'Department',
                        ),
                        child: new DropdownButtonHideUnderline(
                          child: new DropdownButton(
                            isDense: true,
                            onChanged: (newValue) {
                              setState(() {
                                _idDepartment= newValue.toString();
                              });
                            },
                            items: _department.map((item) {
                              return DropdownMenuItem(
                                child: Text(item.value),
                                value: item.id,
                              );
                            }).toList(),
                            value: _idDepartment,
                          ),
                        ),
                      );
                    },
                  ),
                ),