Flutter reset DropdownButton items after select one of them item - flutter

Question

in this simple DropdownButton widget when i select one item, items refreshed and select value is first item of SessionsEntity list items, and i can't select another item,selecting them cause of select first item,
I think after selecting item, that cause of rebuild DropdownButton widget
SessionsEntity sessionData;
BarCodesBloc _barcodesBloc;
...
#override
Widget build(BuildContext context) {
return Scaffold(
key: _scaffoldKey,
appBar: ApplicationAppBar(appBarTitle: sessionData!=null? ' (${sessionData.sessionName}) ':'',),
body: BlocListener(
bloc: _barcodesBloc,
listener: (BuildContext context, BarCodesState state) {
if (state is BarCodeScannedSuccessful) {
player.play('ringtones/2.mp3');
}
if (state is BarCodeScannedError) {
}
if (state is BarCodeScannedDuplicate) {
player.play('ringtones/1.mp3');
}
},
child: BlocBuilder(
bloc: _barcodesBloc,
builder: (BuildContext context, BarCodesState state) {
return FutureBuilder(
future: globals.database.sessions.getAllSessionsFuture(),
builder: (BuildContext context, AsyncSnapshot snapshot) {
if (snapshot.hasData) {
List<SessionsEntity> sessions = snapshot.data;
List<DropdownMenuItem<SessionsEntity>> _dropdownMenuItems;
if (sessions != null && sessions.length > 0) {
_dropdownMenuItems = buildDropdownMenuItems(sessions);
sessionData = _dropdownMenuItems[0].value;
return Stack(
children: <Widget>[
DropdownButtonHideUnderline(
child: Theme(
data: Theme.of(context).copyWith(
canvasColor: Colors.white,
),
child: DropdownButton(
items: _dropdownMenuItems,
isDense: true,
value:sessionData,
onChanged: onChangeDropdownItem,
isExpanded: true,
hint: Text('please select item',
style: Theme.of(context).textTheme.caption.copyWith(color: Colors.black, )),
),
),
),
],
);
} else {
return Container(
child: Center(
child: Text(
Fa.keywords['noAnySessions'],
style: Theme.of(context).textTheme.caption.copyWith(
color: Colors.black,
),
),
),
);
}
} else {
return Container(
child: Center(
child: Text(
Fa.keywords['noAnySessions'],
style: Theme.of(context).textTheme.caption.copyWith(
color: Colors.black,
),
),
),
);
}
},
);
},
),
),
);
}
onChangeDropdownItem(SessionsEntity selectedCompany) {
setState(() {
sessionData = selectedCompany;
print(sessionData.sessionName);
});
}