How to do empty field validation in this TextDropdownFormField dropdown in flutter

Padding( padding: const EdgeInsets.only( left: 10, right: 10, top: 5, bottom: 10), child: TextDropdownFormField( options: ["Male", "Female"], decoration: InputDecoration( border: OutlineInputBorder(), suffixIcon: Icon(Icons.arrow_drop_down), labelText: "Gender"), validator: (value) { if (value!.isEmpty) { return 'Please enter valid gender'; } }, dropdownHeight: 120, ), ),