Flutter app gave an error after performing the necessary integration with laravel api, it returned null

I have a signup page having done all the necessary integration my flutter app still not posting Here I have my signup method

_signup() async {
    var data = {
      'first': firstNameController.text,
      'last': lastNameController.text,
      'email': emailController.text,
      'phone': phoneController.text,
      'age': ageController.dropDownValue!.name,
      'gender': genderController.dropDownValue!.name,
      'password': passwordController.text,
      'confirm': confirmPasswordController.text
    };
    print(data);
    var res = await CallApi().postData(data, 'register');
    print(res);
    var body = json.decode(res.body);
    print(body);

  }

Why I have the api authentication here which is well configured

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

class CallApi {

final String baseURL = 'https://beereapp.edusmart.ng/api/auth/';
  postData(data, apiUrl) async {
    var fullUrl = baseURL + apiUrl;
    return await http.post(
      Uri.parse(fullUrl),
       body: jsonEncode(data),
       headers: _setHeaders());
  }

  getData(apiUrl) async {
    var fullUrl = Uri.parse(baseURL + apiUrl);
    return await http.get(fullUrl, headers: _setHeaders());
  }

  _setHeaders() => {
        'Content-type': 'application/json',
        'Accept': 'application/json',
      };
}

As you can see my terminal below I printed my data out which is showing but next is error. Help please!

  [![My terminal][1]][1] 


  [1]: https://i.stack.imgur.com/10uMa.jpg