guru meditation error when reading data from ESPAsyncWebServer request parameters

I am working on ESP32, New to this device.

I am currently facing the following error. when I try to read the data from request->value().c_str() the serial monitor shows me guru meditation error error SS of error:

My code:

server.on("/register", HTTP_POST, [](AsyncWebServerRequest *request) {
request->send(200, "text/plain", Register(request->getParam("plain",true)).c_str());});

The function:

string Register(AsyncWebParameter *request){

const size_t bufferSize = JSON_OBJECT_SIZE(1) + 250;; StaticJsonDocument data; deserializeJson(data, request->value().c_str()); // facing issue here when request->value().c_str() is called const char* name = data["username"];

return String("Execution complete"); }

I have return simple javascript function to call the API:

function Register() {
const tosend={
      "username":"Mohit",
      "password":"Mohit10#",
      "lastname":"mhatre",
      "firstname":"Mohit"
  };
  const url="http://192.168.2.120/register";
  
  fetch(url, {
    method: 'post',
    headers: {
      // if you want to set
    },
    body: tosend
  })
    .then(
      function(response) {
        response.json().then(function(data) {
          console.log(data);
        });
      }
    )
    .catch(function(err) {
      console.log('Fetch Error :-S', err);
    });
}

Can someone please help me resolve the issue i am new to this and stuck here from yesterday