Flutter : PopUp Form value showing null - flutter

Question

Iam new to flutter.
In my project I need to show a popup when i load my page. So i used WidgetsBindingin initstate().
My code is working fine for showing popup on pageload, but in that popup,there is a textfield to enter mobile number after submitting i need to get that phone number but it not showing the value. While compiling it showing null value.
#override
void initState() {
super.initState();
// _fetchSessionAndNavigate();
WidgetsBinding.instance.addPostFrameCallback((_) async {
await showDialog<String>(
context: context,
builder: (BuildContext context) => new AlertDialog(
content: Stack(
overflow: Overflow.visible,
children: <Widget>[
Positioned(
right: -40.0,
top: -40.0,
child: InkResponse(
onTap: () {
Navigator.of(context).pop();
},
child: CircleAvatar(
child: Icon(Icons.close),
backgroundColor: Colors.red,
),
),
),
Form(
key: _formKey,
child: Column(
mainAxisSize: MainAxisSize.min,
children: <Widget>[
Padding(
padding: EdgeInsets.all(8.0),
child: TextFormField(
keyboardType: TextInputType.number,
maxLines: 1,
obscureText: true,
autofocus: false,
decoration: new InputDecoration(
hintText: 'Enter Mobile Number',
icon: new Icon(
Icons.phone_iphone,
color: Colors.grey,
)),
validator: (value) => value.isEmpty
? 'Phone Number can\'t be empty'
: null,
onSaved: (value) => phoneNumber = value.trim();
),
),
Padding(
padding: const EdgeInsets.all(8.0),
child: RaisedButton(
child: Text("Register Your Device"),
onPressed: () {
if (_formKey.currentState.validate()) {
//_formKey.currentState.save();
Print (_phoneNumber); //NEED TO GET ENTERED VALUE HERE
}
},
),
)
],
),
),
],
),
),
);
});
}

Answer 1

You can remove the await keyword from in front of the function and call it like this:
showDialog({the dialog}).then((dialogResult){
setState((){
yourLocalVariable = dialogResult;
});
});