Flutter: disable DropdownButtonFormField option

I have this widget:

DropdownButtonFormField<String>(
        hint: Text(translate('payments.select_frequency')),
        value: frequency,
        items: frequencies.map((String value) {
          return DropdownMenuItem<String>(
            value: value,
            child: Text(
              translate("expense.$value"),
              style: TextStyle(
                color: disabledFrequencies.contains(value) ? Colors.grey : null,
              ),
            ),
          );
        }).toList(),
        onChanged: (value) async {
          if (!disabledFrequencies.contains(value)) {
            setState(() {
              frequency = value;
            });
          }
        },
        validator: (value) {
          if (value == null) {
            return translate('fill_field');
          }
          return null;
        },
      );

This generates something like this:

Here I should be able to just click the first option but I can select any of them. I opened this issue a while ago in Flutter repo and they mentioned it's not an issue.

What's the option then?

There is enable property on DropdownMenuItem control the tap accessibility.

return DropdownMenuItem<String>(
      value: value,
      enabled: !disabledFrequencies.contains(value), //this
      onTap: () {

Whether or not a user can select this menu item. Defaults to true.