API Call with custom Body in dart flutter

I'm trying to call an API with a custom Body but I always the same error. Error 400 And the API return "can't json/yaml decode the argument"

import 'dart:convert';
import 'package:http/http.dart' as http;
import 'dart:developer' as developer;
Future\<dynamic\> ApiDiolos() async {
final mapHeader = {
'Accept': 'application/json',
'Content-Type': 'application/json',
'X-Diolos-Key':
'My-Custom-API-Key'
};
Uri uri = Uri.parse("https://api.diolos.com/myapipage.php");
var customBody = {};
customBody\["action"\] = "6";
customBody\["limit"\] = "null";
customBody\["offset"\] = "0";
final response = await http.post(uri,
headers: mapHeader,
body: json.encode(customBody.toString()),
encoding: Encoding.getByName('utf-8'));
}
void main() {
ApiDiolos();
}

Everything works fine with postman. I hope you could help my with this issue.

Pass the map variable directly to be encoded:

From

body: json.encode(customBody.toString()),

To

body: json.encode(customBody),

after that make sure the following:

• You are using an authorized API Key.
• You are calling the correct URL, it's preferable to test it before integrating the call within your app.
• Wait for the results of the call.

Look at the following code, and try to refactor it:

Future<Map?> ApiDiolos() async {
  final mapHeader = {
    'Accept': 'application/json',
    'Content-Type': 'application/json',
    'X-Diolos-Key': 'XXXXXXXXXXXXXXXXXXXX' // provide an authorized key
  };
  Uri uri = Uri.parse(
      "https://api.diolos.com/myapipage.php"); // make sure it's a correct URL

  var customBody = {"action": "6", "limit": "null", "offset": "0"};

  final response =
      await http.post(uri, headers: mapHeader, body: json.encode(customBody));

  if (response.statusCode == 200) {
    return json.decode(response.body);
  }
  return null;
}

void main() async {
  print(await ApiDiolos()); // you must wait for the Future to get completed
}