how do i make the controller inside the widget class Input Date extends FormField<String> optional?

class InputDate extends FormField<String> {
 InputDate({
  Key? key,
  TextEditingController? controller,
  FormFieldValidator<String>? validator,
  Function(String?)? onSaved,
  DateTime? initialValue,
  required String label,
 }) : super(
      key: key,
      validator: validator,
      onSaved: onSaved,
      builder: (field) {
        return Column(
          children: [
            TextField(
              controller: controller == null ? controller : newController,
              onTap: () async {
                final DateTime? pickedDate = await showDatePicker(
                  context: field.context,
                  initialDate: DateTime.now(),
                  firstDate: DateTime(
                    1000,
                  ),
                  lastDate: DateTime(1999),
                );
                if (pickedDate != null) {
                  final formattedDate =
                      DateFormat('dd-MM-yyyy').format(pickedDate);
                    controller.text =
                        DateFormat('dd-MM-yyyy').format(formattedDate);
                }
              },

how do i make the controller inside the widget class Input Date extends > FormField optional? can you help

I have tried many times but got lost