How do I solve type "" is not a subtype of type "" in flutter

Issue

I'm quite new to flutter and I'm following a tutorial trying to implement a dropdown widget (code is shown below) in my app. But every time I launch the page, I get this result Exception has occurred. _TypeError (type 'List<DropdownMenuItem>' is not a subtype of type 'List<DropdownMenuItem>?')

Here is a screenshot of my problem

Here is my code

Widget selectDropdown(
    BuildContext context,
    Object initialValue,
    dynamic data,
    Function onChanged, {
      Function onValidate,
    }
  ) {
    return Align(
      alignment: Alignment.topLeft,
      child: Container(
        height: 75,
        width: 100,
        padding: const EdgeInsets.only(top: 5),
        child: DropdownButtonFormField(
          hint: const Text(
            "Select",
            style: TextStyle(
              color: Colors.white
            )
          ),
          isDense: true,
          value:  null,
          decoration: fieldDecoration(context, "", ""),
          onChanged: (VariableProduct newValue) {
            FocusScope.of(context).requestFocus(FocusNode());
            onChanged(newValue);
          },
          validator: (value) {
            return onValidate(value);
          },
          items: data != null ?
            data.map<DropdownMenuItem<VariableProduct>>(
              (VariableProduct data) {
                return DropdownMenuItem<VariableProduct>(
                  value: data,
                  child: Text(
                    data.attributes.first.option + " " + data.attributes.first.name,
                    style: const TextStyle(color: Palette.backgroundColor)
                  ),
                );
              }
            ).toList()
            : null,
        )
      ),
    );
  }

I will be glad if I can be helped. Thank you

Solution

Add type parameter to DropdownButtonFormField:

DropdownButtonFormField<VariableProduct>

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