Hi in the below code when I am hit the signin button showing below error ,below details are passing as body I am able to print the data but response is not coming anything.

Enter mobile number and password able to see the response in postman but from app not getting any error.

Can any one help me where I did the mistake

{Mobile: 8250030771, Password: Test@123, Otp: N, Type: Patient}
Error: type 'Null' is not a 'bool' in boolean expression

SignIn.dart:

class SignIn extends StatefulWidget {
  @override
  _SignInState createState() => _SignInState();
}

class _SignInState extends State<SignIn> {
  final _formKey = GlobalKey<FormState>();
 late String mobile, password,otp,type;
  bool isLoading=false;
  TextEditingController _mobileController=new TextEditingController();
  TextEditingController _passwordController=new TextEditingController();
  TextEditingController _otpController=new TextEditingController();
  GlobalKey<ScaffoldState>_scaffoldKey=GlobalKey();
  late ScaffoldMessengerState scaffoldMessenger ;
  @override
  Widget build(BuildContext context) {
    scaffoldMessenger = ScaffoldMessenger.of(context);
    return Scaffold(
      key: _scaffoldKey,
        body: SingleChildScrollView(
      child: Container(
        width: MediaQuery.of(context).size.width,
        height: MediaQuery.of(context).size.height,
        child: Stack(
          children: <Widget>[
            Container(
              width: double.infinity,
              height: double.infinity,
              child: Image.asset(
                "assets/background.jpg",
                fit: BoxFit.fill,
              ),
            ),
            Container(
              child: Column(
                crossAxisAlignment: CrossAxisAlignment.center,
                mainAxisAlignment: MainAxisAlignment.center,
                children: <Widget>[
                  Center(
                      child: Image.asset(
                    "assets/logo.png",
                    height: 30,
                    width: 30,
                    alignment: Alignment.center,
                  )),
                  SizedBox(
                    height: 13,
                  ),
                  Text(
                    "Learn With Us",
                    style: GoogleFonts.roboto(
                        textStyle: TextStyle(
                            fontSize: 27,
                            color: Colors.white,
                            letterSpacing: 1)),
                  ),
                  SizedBox(
                    height: 5,
                  ),
                  Container(
                    width: 180,
                    child: Text(
                      "RRTutors, Hyderabad",
                      textAlign: TextAlign.center,
                      style: GoogleFonts.roboto(
                        textStyle: TextStyle(
                            color: Colors.white54,
                            letterSpacing: 0.6,
                            fontSize: 11),
                      ),
                    ),
                  ),
                  SizedBox(
                    height: 40,
                  ),
                  Text(
                    "Sign In",
                    textAlign: TextAlign.center,
                    style: GoogleFonts.roboto(
                      textStyle: TextStyle(
                        color: Colors.white,
                        letterSpacing: 1,
                        fontSize: 23,
                      ),
                    ),
                  ),
                  SizedBox(
                    height: 8,
                  ),
                  Row(
                    mainAxisAlignment: MainAxisAlignment.center,
                    children: <Widget>[
                      Text(
                        "Learn new Technologies