Boolean Expression must not be null error

login_page.dart

import 'dart:async';

import 'package:flutter/material.dart';
import 'package:html/dom.dart' as dom;
import 'package:html/parser.dart';
import 'package:http/http.dart';
import '../live_kh.dart';
import 'package:flutter/cupertino.dart';
import 'hall_page.dart';

class LoginPage extends StatefulWidget {
  final String congregationName;
  final String congregationId;
  final String congregationUrl;
  final String imgSrc;
  final LiveKH liveKH;

  const LoginPage(
      {Key key,
      @required this.congregationName,
      @required this.congregationId,
      @required this.imgSrc,
      @required this.liveKH,
      @required this.congregationUrl})
      : super(key: key);

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

class _LoginPageState extends State<LoginPage> {
  Color loginButtonColor = Colors.grey;
  TextEditingController _listPassController = TextEditingController();
  int dropDownButtonValue = 1;

  bool _listPassFieldIsValid = false;

  @override
  void initState() {
    debugPrint("Logging into ${widget.congregationName}");
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    double screenWidth = MediaQuery.of(context).size.width;
    double screenHeight = MediaQuery.of(context).size.height;

    return Scaffold(
      backgroundColor: Colors.white,
      appBar: AppBar(
        backgroundColor: Theme.of(context).primaryColor,
        title: Text("Login"),
        centerTitle: true,
      ),
      body: Center(
        child: OrientationBuilder(builder: (BuildContext context, orientation) {
          return Card(
            elevation: 10.0,
            child: Container(
              width: (orientation == Orientation.portrait)
                  ? screenWidth
                  : screenHeight,
              height: (orientation == Orientation.portrait)
                  ? screenHeight / 1.75
                  : screenWidth / 2,
              child: Column(
                mainAxisAlignment: MainAxisAlignment.center,
                children: <Widget>[
                  Hero(
                    tag: widget.congregationName,
                    child: Card(
                      elevation: 4.0,
                      shape: CircleBorder(),
                      child: Image.network(
                        widget.imgSrc,
                        fit: BoxFit.fill,
                        width: (orientation == Orientation.portrait)
                            ? (screenWidth / 2) // if it's portrait
                            : (screenHeight / 4), // if it's landscape
                      ),
                    ),
                  ),
                  Padding(
                    padding: EdgeInsets.only(top: 10.0),
                  ),
                  Text(
                    "${widget.congregationName}",
                    textAlign: TextAlign.center,
                    style:
                        TextStyle(fontWeight: FontWeight.w200, fontSize: 20.0),
                  ),
                  Padding(
                    padding: const EdgeInsets.symmetric(
                        vertical: 8.0, horizontal: 30.0),
                    child: Column(
                      crossAxisAlignment: CrossAxisAlignment.start,
                      children: <Widget>[
                        TextField(
                          controller: _listPassController,
                          onChanged: (value) {
                            if (value.length > 0) {
                              setState(() {
                                _listPassFieldIsValid = true;
                              });
                            } else if (_listPassFieldIsValid &&
                                value.length == 0) {
                              setState(() {
                                _listPassFieldIsValid = false;
                              });
                            }
                            debugPrint(_listPassFieldIsValid.toString());
                          },
                          keyboardType: TextInputType.number,
                          maxLength: 4,
                          decoration: InputDecoration(hintText: "Enter PIN"),
                        ),
                        Padding(
                            padding: EdgeInsets.symmetric(
                                horizontal: screenWidth / 10)),
                        Row(
                          children: <Widget>[
                            new Text("Enter the number of listeners: "),
                            Padding(
                                padding:
                                    EdgeInsets.symmetric(horizontal: 10.0)),
                            DropdownButton<int>(
                              value: dropDownButtonValue,
                              isDense: true,
                              items: new List<DropdownMenuItem<int>>.generate(
                                  10,
                                  (i) => DropdownMenuItem<int>(
                                        value: i,
                                        child: Text("$i"),
                                      )),
                              onChanged: (int value) {
                                setState(() {
                                  dropDownButtonValue = value;
                                });
                              },
                            ),
                          ],
                        ),
                      ],
                    ),
                  ),
                  Padding(
                    padding: EdgeInsets.all(10.0),
                  ),
                  RaisedButton(
                      color: Theme.of(context).primaryColor,
                      child: Text("Login"),
                      onPressed: !_listPassFieldIsValid
                          ? null
                          : () =>
                               login(
                                  listPass: _listPassController.text,
                                  numOfListeners: dropDownButtonValue)
                            )
                ],
              ),
            ),
            color: Colors.white,
          );
        }),
      ),
    );
  }

  Future login(
      {@required String listPass, @required int numOfListeners}) async {
    debugPrint('Login Button Pressed.');
    if (_listPassFieldIsValid) {
      debugPrint("Login Field is valid");
      assert(widget.congregationName != null);
      assert(widget.congregationUrl != null);
      assert(widget.congregationId != null);
      assert(listPass != null);
      assert(numOfListeners != null);
      debugPrint("Assertions Passed");
      debugPrint("ListPass: $listPass");
      debugPrint("NumOfListeners: $numOfListeners");

      Map loginResult = await widget.liveKH.login(
          congregationId: widget.congregationId,
          congregationName: widget.congregationName,
          congregationUrl: widget.congregationUrl,
          listPass: listPass);
      if (loginResult['success']) {
        String body = loginResult['response']?.body;

        await Navigator
            .of(context)
            .push(new MaterialPageRoute(builder: (BuildContext context) {
          return HallPage(
            title: widget.congregationName,
            body: body,
          );
        }));

        widget.liveKH.logout();
      } else {
        showErrorSnackbar(context);
      }
    }
  }

  void showErrorSnackbar(BuildContext context) {
    SnackBar snackBar = SnackBar(
        content:
            Text("Login failed. Please check your password and try again"));
    Scaffold.of(context).showSnackBar(snackBar);
  }
}

Stacktrace:
E/flutter ( 6830): [ERROR:topaz/lib/tonic/logging/dart_error.cc(16)] Unhandled exception:
E/flutter ( 6830): Failed assertion: boolean expression must not be null
E/flutter ( 6830): #0 _LoginPageState.login (file:///home/thinkdigital/IdeaProjects/livekh_flutter/lib/ui/login_page.dart)
E/flutter ( 6830):
E/flutter ( 6830): #1 _LoginPageState.build.. (file:///home/thinkdigital/IdeaProjects/livekh_flutter/lib/ui/login_page.dart:156:32) Shows error here on line 156, but the real error was on line 189 `loginResult['success']' should be loginResult['status']
E/flutter ( 6830): #2 _InkResponseState._handleTap (package:flutter/src/material/ink_well.dart:494:14)
E/flutter ( 6830): #3 _InkResponseState.build. (package:flutter/src/material/ink_well.dart:549:30)
E/flutter ( 6830): #4 GestureRecognizer.invokeCallback (package:flutter/src/gestures/recognizer.dart:102:24)
E/flutter ( 6830): #5 TapGestureRecognizer._checkUp (package:flutter/src/gestures/tap.dart:161:9)
E/flutter ( 6830): #6 TapGestureRecognizer.handlePrimaryPointer (package:flutter/src/gestures/tap.dart:94:7)
E/flutter ( 6830): #7 PrimaryPointerGestureRecognizer.handleEvent (package:flutter/src/gestures/recognizer.dart:315:9)
E/flutter ( 6830): #8 PointerRouter._dispatch (package:flutter/src/gestures/pointer_router.dart:73:12)
E/flutter ( 6830): #9 PointerRouter.route (package:flutter/src/gestures/pointer_router.dart:101:11)
E/flutter ( 6830): #10 _WidgetsFlutterBinding&BindingBase&GestureBinding.handleEvent (package:flutter/src/gestures/binding.dart:143:19)
E/flutter ( 6830): #11 _WidgetsFlutterBinding&BindingBase&GestureBinding.dispatchEvent (package:flutter/src/gestures/binding.dart:121:22)
E/flutter ( 6830): #12 _WidgetsFlutterBinding&BindingBase&GestureBinding._handlePointerEvent (package:flutter/src/gestures/binding.dart:101:7)
E/flutter ( 6830): #13 _WidgetsFlutterBinding&BindingBase&GestureBinding._flushPointerEventQueue (package:flutter/src/gestures/binding.dart:64:7)
E/flutter ( 6830): #14 _WidgetsFlutterBinding&BindingBase&GestureBinding._handlePointerDataPacket (package:flutter/src/gestures/binding.dart:48:7)
E/flutter ( 6830): #15 _invoke1 (dart:ui/hooks.dart:134:13)
E/flutter ( 6830): #16 _dispatchPointerDataPacket (dart:ui/hooks.dart:91:5)