fixed.ino

#include <ESPAsyncTCP.h>
#include <Ticker.h>
#include <ESPAsyncWebServer.h>
#include <ESP8266WiFi.h>
#include <functional>

using namespace std;
using namespace placeholders;

class MyServer : public AsyncWebServer {
public:

  MyServer(int port) : AsyncWebServer(port) {
  } 

  void begin() {
    _periodicTrigger.attach(1.0f, &MyServer::s_periodicTask, static_cast<void*>(this));

    on("/index.html", std::bind(&MyServer::indexHandler, this, _1));

    AsyncWebServer::begin();
  }
protected:
  Ticker _periodicTrigger;
  int value = 0;

  void indexHandler(AsyncWebServerRequest* request) {
    String text = "Value is " + value;
    request->send(200,"text/html",text);
  }

  void periodicTask() {
    Serial.printf("Periodic task. Result = %d\n", value);
    value++;
  }

  static void s_periodicTask(void* arg) {
    MyServer* self = reinterpret_cast<MyServer*>(arg);
    self->periodicTask();
  }

};

MyServer server(80);

void additionalHandler(AsyncWebServerRequest* request) {
  request->send(200, "text/html", "normal handler");
}

void setup()
{
  //Error(active)  function "AsyncWebServer::on(const char *uri, ArRequestHandlerFunction onRequest)" (declared at line 372 of ESPAsyncWebServer.h is inaccessible  CallbackTest.ino 55
  server.on("/other.html", additionalHandler);
  server.begin();

}

void loop()
{
}