Query in button error on the first click flutter - flutter

Question

I have problem in button to search user by phone number. When I press button for the first time, query can't retrieve the data, like this : Debug Console 1. But if I press button again for the second time or more, query can retrieve the data, like this : Debug Console 2.
This is my button code :
Widget tmblKonfirm() {
return Center(
child: Container(
decoration: BoxDecoration(
borderRadius: BorderRadius.circular(10),
),
child: FlatButton(
onPressed: () async{
await pr.show();
print('NoHP : ' + nohp);
final dataUser = FirebaseDatabase.instance
.reference()
.child("users")
.orderByChild("pengguna_nomor")
.equalTo(nohp);
print('Data User : ' + dataUser.toString());
dataUser.once().then((DataSnapshot snapshot) {
Map<dynamic, dynamic> data = snapshot.value;
setState(() {
testUser = snapshot.value.toString();
});
data.forEach((key, values) {
lists.add(values);
});
});
print('User 1 : ' + lists.toString());
pr.hide();
if (_formKeyKonfirm.currentState.validate()) {
print('User 2 : ' + testUser.toString());
if (testUser != 'null'){
// setState(() {
sttsUser = true;
// });
// showWidgetTopup();
}
else {
showDialog(
context: context,
builder: (context) {
return AlertDialog(
content: Text(
'Nomor HP tidak terdaftar'),
actions: <Widget>[
FlatButton(
onPressed: () => Navigator.of(context).pop(),
child: Text('OK'),
)
],
);
},
);
}
}
},
child: Text('Konfirmasi',
style: TextStyle(color: Colors.black, fontSize: 25)),
color: Colors.lightBlue,
),
));
}

Answer 1

dataUser is a Query so you always have to fetch the data snapshot (once() or onValue)
Instead of dataUser.once().then use await dataUser.once(), otherwise the snapshot callback can be actually called after the onPressed function finishes