Problem with establishing classic bluetooth connection

Szymeski97

Hello everyone.
I need to write an app using Bluetooth to data flow. My experience with QT is not very big and I have never used Bluetooth classes before, so I wanted to learn this from some simple example. I chose my BT speaker and wrote some lines of code which should connect my RaspberryPi to the speaker. Unfortunately I get errors I can't deal with.

Console output:

pi@raspberrypi:~/Desktop/build-MyApp-Desktop-Debug $ sudo ./MyApp
libEGL warning: DRI2: failed to authenticate
QStandardPaths: XDG_RUNTIME_DIR not set, defaulting to '/tmp/runtime-root'
Bluetooth works! 

New device found:  "40-1A-94-0C-4D-18"  (  "40:1A:94:0C:4D:18"  )
New device found:  "4B-BB-97-31-66-6A"  (  "4B:BB:97:31:66:6A"  )
New device found:  "4F-C3-B4-14-74-64"  (  "4F:C3:B4:14:74:64"  )
New device found:  "59-42-47-87-38-EA"  (  "59:42:47:87:38:EA"  )
New device found:  "A10"  (  "70:78:1A:FF:4C:02"  )
qt.bluetooth.bluez: SDP scan failure QProcess::ExitStatus(CrashExit) 9
qt.bluetooth.bluez: SDP search failed for "<Unknown>"

This error is troublesome for me because I don't understand why it appears and how to fix it.
Also, when I add this code line after calling connectToService():

qDebug() << socket.error() << endl;

I gets something like that:

QBluetoothSocket::SocketError( -2 )

Which means: QBluetoothSocket::NoSocketError as far as I know, but I can't interpret it properly so it isn't a helping hand.

My Class source file:

#include "btconnection.h"
#include <QDebug>
#include <QObject>
#include <QtBluetooth/QBluetoothDeviceInfo>
#include <QtBluetooth/QBluetoothDeviceDiscoveryAgent>
#include <QtBluetooth/QBluetoothUuid>
#include <QtBluetooth/QBluetoothSocket>

BTConnection::BTConnection(QObject *parent) : QObject(parent)
{

}

void BTConnection::init()
{
    localDevice = new QBluetoothLocalDevice(this);
    if(localDevice->isValid())
    {
        qInfo() << "Bluetooth works!" <<endl;
        localDevice->powerOn();
        localDevice->setHostMode(QBluetoothLocalDevice::HostDiscoverable);
        this->startDeviceDiscovery();
    }
    else
    {
        qInfo() << "Bluetooth does't works!" <<endl;
    }
}

void BTConnection::startDeviceDiscovery()
{
    deviceAgent = new QBluetoothDeviceDiscoveryAgent(this);
    QObject::connect(deviceAgent, SIGNAL(deviceDiscovered(QBluetoothDeviceInfo)), this, SLOT(deviceDiscovered(QBluetoothDeviceInfo)));
    deviceAgent->start();
}

void BTConnection::deviceDiscovered(const QBluetoothDeviceInfo &device)
{
    qInfo()<<"New device found: "<<device.name()<<" ( "<<device.address().toString()<<" )";
    if(device.name()=="A10")
    {
        deviceAgent->stop();
        localDevice->requestPairing(device.address(), QBluetoothLocalDevice::Paired);
        QBluetoothSocket socket(QBluetoothServiceInfo::RfcommProtocol);
        QString uuidString = "{0000110e-0000-1000-8000-00805f9b34fb}"; // UUID of the 'A/V Remote Control' service
        QBluetoothUuid uuid(uuidString);
        socket.connectToService(device.address(), uuid, QIODevice::ReadWrite);
    }
}

BTConnection::~BTConnection()
{
    if(!deviceAgent)
        delete deviceAgent;
    if(localDevice!=nullptr)
        delete localDevice;
}

.PRO file

#-------------------------------------------------
#
# Project created by QtCreator 2019-11-10T22:42:54
#
#-------------------------------------------------

QT       += core gui bluetooth

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

TARGET = MyApp
TEMPLATE = app

# The following define makes your compiler emit warnings if you use
# any feature of Qt which as been marked as deprecated (the exact warnings
# depend on your compiler). Please consult the documentation of the
# deprecated API in order to know how to port your code away from it.
DEFINES += QT_DEPRECATED_WARNINGS

# You can also make your code fail to compile if you use deprecated APIs.
# In order to do so, uncomment the following line.
# You can also select to disable deprecated APIs only up to a certain version of Qt.
#DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0x060000    # disables all the APIs deprecated before Qt 6.0.0


SOURCES += main.cpp\
        mainwindow.cpp \
    btconnection.cpp

HEADERS  += mainwindow.h \
    btconnection.h

FORMS    += mainwindow.ui

If it will be helpful I can add that when I tried to use another form of "ConnectToService()" method:

connectToService(const QBluetoothServiceInfo &service, QIODevice::OpenMode openMode = ReadWrite)

I also couldn't establish a connection, and trying to read UUID of any service gave it:
00000000-0000-0000-0000-000000000000

I would appreciate some tips or any help because I have been looking for a solution for a long time and found nothing.

Version of my RPi is 3B+ Rev 1.3 , Raspbian version is 10 (buster), QT version is 5.11.3