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,

);

}