The following _CastError was thrown while handling a gesture: Null check operator used on a null value

So i made an Create methode for my Sqlapi. I made a test project where the create method worked. So now i try to implement it in to the real application but now i get some strange error (Shown in the image). Any help would be amazing.

Error:

All the containers are the same.
My flutter code:

import 'package:flutter/material.dart';
import 'package:schepp/main.dart';
import '../Data_provider/api_service.dart';
import '../Model/KlantModel.dart';

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  static const appTitle = 'Inloggen';

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      title: appTitle,
      home: Registerenpage(
          title: appTitle
      ),
    );
  }
}

class Registerenpage extends StatefulWidget {
  const Registerenpage({Key? key, required this.title}) : super(key: key);

  final String title;

  @override
  _RegisterenpageState createState() => _RegisterenpageState();
}

class _RegisterenpageState extends State<Registerenpage> {
  _RegisterenpageState();

  final ApiService api = ApiService();
  final _addFormKey = GlobalKey<FormState>();
  final _mailaddres = TextEditingController();
  final _wachtwoord = TextEditingController();
  final _klantvoornaam = TextEditingController();
  final _tussenvoegsel = TextEditingController();
  final _klantachternaam = TextEditingController();
  final _bedrijfsnaam = TextEditingController();
  final _telefoonnummer = TextEditingController();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.black,
      body: SingleChildScrollView(
        child: Column(
          key: _addFormKey,
            mainAxisSize: MainAxisSize.max,
            mainAxisAlignment: MainAxisAlignment.center,
            children:<Widget>[
              Container(
                margin: const EdgeInsets.only(left: 10.0, right: 10.0, top:80.0),
                child: const Text("Registeren",style: TextStyle(color:Colors.white,fontSize:20),),
              ),
              Container(),
              Container(),  
              Container(),  
              Container(),
              Container(
               margin: const EdgeInsets.only(left: 50.0, right: 50.0),
                child: Column(
                 children: <Widget>[
                   const Text('Mailaddres'),
                   TextFormField(
                    controller: _mailaddres,
                    decoration: const InputDecoration(
                     border: UnderlineInputBorder(),
                      enabledBorder: UnderlineInputBorder(
                        borderSide: BorderSide(color: Colors.white),
                      ),
                      focusedBorder: UnderlineInputBorder(
                        borderSide: BorderSide(color: Colors.white),
                      ),
                      labelText: 'Mailaddress *', labelStyle: TextStyle(color:Colors.white),
                      ),
                    style: const TextStyle(color: Colors.white, fontSize: 16.0,),
                    validator: (value) {
                      if (value!.isEmpty) {
                        return 'Please enter mailaddres';
                      }
                      return null;
                      },
                    onChanged: (value) {},
                   ),
                 ],
                ),
              ),
              Container(), 
              Container(),
              Container(
                padding: const EdgeInsets.all(20.0),
                child: Column(
                  children: <Widget>[
                    ElevatedButton(
                      child: const Text('Registeren', style: TextStyle(color:Colors.white,fontSize:16)),
                      onPressed: () {
                        if (_addFormKey.currentState!.validate()) {
                          _addFormKey.currentState!.save();
                          api.createCase(Cases(mailaddres: _mailaddres.text, wachtwoord: _wachtwoord.text,
                              klantvoornaam: _klantvoornaam.text, tussenvoegsel: _tussenvoegsel.text,
                              klantachternaam: _klantachternaam.text, bedrijfsnaam: _bedrijfsnaam.text,
                              telefoonnummer: _telefoonnummer.text));

                          Navigator.push(
                              context,
                              MaterialPageRoute(
                              builder: (context) => Loginpagina(title: widget.title),
                              ),
                          );
                        }
                        },
                      style: ElevatedButton.styleFrom(
                        primary: Colors.deepOrange,
                        padding: const EdgeInsets.all(20),
                      ),
                    )
                  ],
                ),
              ),
            ]
        ),
      ),
    );
  }
}

This is my KlantModel:

import 'dart:convert';

List<Cases> welcomeFromJson(String str) => List<Cases>.from(json.decode(str).map((x) => Cases.fromJson(x)));

String welcomeToJson(List<Cases> data) => json.encode(List<dynamic>.from(data.map((x) => x.toJson())));

class Cases {
  Cases({
    this.klantId,
    this.mailaddres,
    this.wachtwoord,
    this.klantvoornaam,
    this.tussenvoegsel,
    this.klantachternaam,
    this.bedrijfsnaam,
    this.telefoonnummer,
  });

  int? klantId;
  String? mailaddres;
  String? wachtwoord;
  String? klantvoornaam;
  String? tussenvoegsel;
  String? klantachternaam;
  String? bedrijfsnaam;
  String? telefoonnummer;

  factory Cases.fromJson(Map<String, dynamic> json) => Cases(
    klantId: json["KlantId"],
    mailaddres: json["Mailaddres"],
    wachtwoord: json["Wachtwoord"],
    klantvoornaam: json["Klantvoornaam"],
    tussenvoegsel: json["Tussenvoegsel"],
    klantachternaam: json["Klantachternaam"],
    bedrijfsnaam: json["Bedrijfsnaam"],
    telefoonnummer: json["Telefoonnummer"],
  );

  Map<String, dynamic> toJson() => {
    "KlantId": klantId,
    "Mailaddres": mailaddres,
    "Wachtwoord": wachtwoord,
    "Klantvoornaam": klantvoornaam,
    "Tussenvoegsel": tussenvoegsel,
    "Klantachternaam": klantachternaam,
    "Bedrijfsnaam": bedrijfsnaam,
    "Telefoonnummer": telefoonnummer,
  };
}

——–! UPDATE ——–

I get this error when i press on the ElevatedButton in the last container. i’am trying to sent the information to another dart file which updates it to a rest api. if i’m correct it gets stuck at the if (_addFormKey.currentState!.validate())

>Solution :

You are not using Form widget. Wrap your Column with Form and use the _addFormKey on Form widget instead of Column.

 child: Form(
  key: _addFormKey,
  child: Column(

And validator

validator: (value) {
  if (value == null || value.isEmpty) {
    return 'Please enter mailaddres';
  }
  return null;
},

And

onPressed: () {
  final isValidate = _addFormKey.currentState?.validate();

  if (isValidate == null) {
    print("got Null isValidate");
    return;
  }
  if (isValidate) {
    _addFormKey.currentState!.save();

More about Form