value == null || items.where((DropdownMenuItem item) => item.value == value).length == 1

I/flutter (23889): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
I/flutter (23889): The following assertion was thrown building DropdownButtonFormField<dynamic>(dirty, state:
I/flutter (23889): _DropdownButtonFormFieldState<dynamic>#94f46):
I/flutter (23889): 'package:flutter/src/material/dropdown.dart': Failed assertion: line 514 pos 15: 'items == null ||
I/flutter (23889): value == null || items.where((DropdownMenuItem<T> item) => item.value == value).length == 1': is not
I/flutter (23889): true.


........................'

use init state to initialize value part in

buildDropdownButtonFormField

void initState() {

String x = controller.text; //on update initial value

x = x.trim();

if (x != "" && x != null) {

selectBoxvalue = x;

}

super.initState();

}

buildDropdownButtonFormField(BuildContext context) {

return DropdownButtonFormField(

items: dropDownList.toList(),

decoration: InputDecoration(

labelText: Translations.of(context).text(label),

),

onChanged: (value) {

setState(() {

selectBoxvalue = value;

});

if (controller != null) {

controller.text = value;

}

if (onChangeSelectMap != null) {

onChangeSelectMap(value);

}

},

value: selectBoxvalue ?? null,

);

}

select box flutter

FormField(
        initialValue: initialValue,
        builder: (FormFieldState state) {
          return InputDecorator(
            decoration: InputDecoration(
              labelText: Translations.of(context).text(label),
            ),
            child: DropdownButtonHideUnderline(
              child: DropdownButton(
                value: selectBoxvalue ?? null,
                items: dropDownList.toList(),
                onChanged: (value) {
                  setState(() {
                    selectBoxvalue = value;
                  });

                  if (controller != null) {
                    controller.text = value;
                  }

                  if (onChangeSelectMap != null) {
                    onChangeSelectMap(value);
                  }
                },
              ),
            ),
          );
        });


///////////////////////////

map.forEach((k, v) {
      dropDownList.add(DropdownMenuItem(
        value: k,
        child: new Text(v),
      ));
    });
////////////////////////////////