Unhandled Exception: NoSuchMethodError: The getter "int variable" was called on null. E/flutter ( 2129): Receiver: null E/flutter ( 2129): Tried calling: "int variable"

[vinayvishnu725]

am getting otp was called null why because i dono... could anybody suggest me

String otpvalue ;
String otpvalue1 = "1234";

void otpvalidateandsubmit() async {
if (otpvalidateandsave()) {
if (postResult.otp == otpvalue) {
Navigator.of(context).push(
MaterialPageRoute(builder: (context) => Home()),
);
}
}
}

and another page
is below

import 'dart:convert';
import 'dart:async';

import 'package:http/http.dart' as http;

class PostResult {
String status;
String message;
var otp;

PostResult({this.status, this.message, this.otp});

factory PostResult.createPostResult(Map<String, dynamic> object) {
return PostResult(
status: object['status'],
message: object['message'],
otp: object['otp'],

);

}
static Future connectToAPI(String name, String phone, String referal, String type) async {
String apiURL = "http://xx2.1xx.1.x:8xx0/api/user/register";
print(name + phone + referal + type);

var apiResult = await http.post(apiURL,headers:{"Content-Type": "application/json"}, body: json.encode({"name": name, "phone": phone, "referral_code":referal, "register_type":type}));

var jsonObject = json.decode(apiResult.body);
print(jsonObject);

return PostResult.createPostResult(jsonObject); 

}

}