The element type 'List<DropdownMenuItem<double>>' can't be assigned to the list type 'DropdownMenuItem<double>'

I'm having a problem when ı try to automatize my Items which is a part of DropDownButton. Every time ı try to execute the function below ı get this error:The element type 'List<DropdownMenuItem>' can't be assigned to the list type 'DropdownMenuItem'.

I tried not to use map function and did the same as Flutter's official page that describes how to use a dropdown button but nothing has changed. I got the same error.

class DataHelper {
  static List<String> tumDerslerinHarflerii() {
    return ["AA", "BA", "BB", "CB", "CC", "CD", "DD", "FD", "FF"];
  }

  static double? harfiNotaCevir(String harf) {
    switch (harf) {
      case "AA":
        return 4;
      case "BA":
        return 3.5;
      case "BB":
        return 3.0;
      case "CB":
        return 2.5;
      case "CC":
        return 2;
      case "DC":
        return 1.5;
      case "DD":
        return 1.0;
      case "FD":
        return 0.5;
      case "FF":
        return 0.0;
    }
    return null;
  }

  static List<DropdownMenuItem<double>> tumDerslerinHarfleri() {
    return tumDerslerinHarflerii()
        .map(
          (e) => DropdownMenuItem(
            child: Text("$e"),
            value: harfiNotaCevir(e),
          ),
        )
        .toList();
  }
}

And I'm using it in my DropDownWidget:

_buildHarfler(GlobalKey buildHarfKey, double _value) {
    return Container(
      decoration: BoxDecoration(
        color: Sabitler.anaRenk.withOpacity(0.3),
        borderRadius: Sabitler.borderRadius,
      ),
      child: DropdownButton<double>(
        key: buildHarfKey,
        value: _value,
        items: [
          // Dropdown içindeki elementlerin her biri bir widget; liste istiyor.
          DataHelper.tumDerslerinHarfleri(),
        ],
        onChanged: (secilenDeger) {
          _value = secilenDeger!;
        },
      ),
    );
  }