Flutter: keyboard disappears immediately when editing my text fields

I'm trying to build an app front end with flutter and it's my first time so I faced some bugs like this one: there is no way to edit my textformfield because I putted my form elements in a listview ! when keyboard appears to enter some text to the field it disappears in a second ! I need an immediat solution please :(

import 'package:flutter/material.dart';
import'package:dubai274_app/mobile_verif.dart';
import 'EnsureVisible.dart';
class SignUp extends StatefulWidget{
  static String tag = 'Sign_up-page';
  SignUp_Page createState() => SignUp_Page();

}
class SignUp_Page extends State<SignUp>{
  


  List<DropdownMenuItem<String>> _Dropdownmenuitems;
  String _statusSel;
  List<DropdownMenuItem<String>> _getDropdownmenuitem(){

    List<DropdownMenuItem<String>> items=new List();
    items.add(new DropdownMenuItem(value:'Emirates',child: new Text('United Arab Emirates')));
    items.add(new DropdownMenuItem(value:'Tun',child: new Text('Tunisia')));
    return items;
  }
  void changeddropdowselecteditem(String selecteditem){
    setState(() {
_statusSel=selecteditem;
    });
  }
   @override
  void initState() {
    // TODO: implement initState
    //listViewController=new ScrollController().addListener(_scrollListener);
    _Dropdownmenuitems=_getDropdownmenuitem();
    _statusSel=_Dropdownmenuitems[0].value;
  }
@override
  Widget build(BuildContext context) {
  final scaffoldKey = GlobalKey<ScaffoldState>();
  final formKey = GlobalKey<FormState>();
  final TextEditingController _controller = new TextEditingController();
  
final first_value=TextFormField(autofocus: false,
  validator: (val) =>
  val.length < 6 ? 'First name required' : null,
  decoration: InputDecoration(
    labelText: 'First Name',
    hintText: 'First Name',
    contentPadding: EdgeInsets.fromLTRB(20.0, 10.0, 20.0, 10.0),


  ),);

final family_value=TextFormField(autofocus: false,
  decoration: InputDecoration(
    labelText: 'Last Name',
    hintText: 'Last Name',
    contentPadding: EdgeInsets.fromLTRB(20.0, 10.0, 20.0, 10.0),


  ),);

final Nationality=new DropdownButton(items: _Dropdownmenuitems, value:_statusSel,onChanged: changeddropdowselecteditem);

final email_value=TextFormField(keyboardType: TextInputType.emailAddress,
  autofocus: false,
  decoration: InputDecoration(
    labelText: 'Email',
    hintText: 'Email',
    contentPadding: EdgeInsets.fromLTRB(20.0, 10.0, 20.0, 10.0),

  ),);

final password=Column(children: <Widget>[ TextFormField(autofocus: false,

  obscureText: true,
  decoration: InputDecoration(
    labelText: 'Password',
    hintText: 'Password',
    contentPadding: EdgeInsets.fromLTRB(20.0, 10.0, 20.0, 10.0),)), Text('Min 8 characters with at least one special character')]);
  void _performsignup() {

    final snackbar = SnackBar(
      content: Text('Email: $email, password: $password'),
    );

    scaffoldKey.currentState.showSnackBar(snackbar);
  }
  void _submit() {
    final form = formKey.currentState;

    if (form.validate()) {
      form.save();
      Navigator.push(
        context,
        MaterialPageRoute(builder: (context) => MobileVerif()),
      );

      _performsignup();
    }
  }
final forward_signedin=FloatingActionButton(tooltip: 'Go forward',
  child: Icon(Icons.arrow_forward,color: Colors.white,size: 38.4,),


  onPressed: (){_submit();},);

return MaterialApp(
      title: 'Sign_Up_Page',
      home: Scaffold(
        appBar: AppBar(
          elevation: 0.0,
      backgroundColor: Colors.transparent,
      title: const Text('Sign Up',style: TextStyle(color: Colors.blueAccent, fontSize: 25.0,fontWeight: FontWeight.bold),textAlign: TextAlign.center,),
  centerTitle: true,
  leading: IconButton(
  tooltip: 'Previous choice',
  icon: const Icon(Icons.arrow_back_ios),
    onPressed: () { Navigator.pop(context);},
    color: Colors.black,
    iconSize: 20.0,
  ),
  ),
        body: new Container(
          decoration: new BoxDecoration(
          image: new DecorationImage(
            image: new AssetImage("assets/background.png"),
            fit: BoxFit.cover,
          ),
        ),
        child: new Form( key: formKey,
            child: new Padding( padding: new EdgeInsets.all(40.0), child: ListView(
      children: <Widget>[

       first_value,
       family_value,
       Nationality,
       email_value,
       password,
       new ListTile( title:forward_signedin,)],
)) )

        ),


        ),

      );


  }
}

You have the following code in your build function:

final formKey = GlobalKey<FormState>();

This is the problem. You have to either make it static or move to initState()