Create and show the object of my QWidget (heir) in the main window (heir of QMainWindow)

[Algorithm0]

I am new in Qt, so I ask you for help. I also warn that my English can be bad. My problem:
I created an heir from QWidget. Separately, the objects of this class work the way I want. If I instantiate this class and call show().
I want to divide my main window into four parts, so that in each of the parts my widget (video stream) works. But at the same time I get an empty window. Here are my files:

#ifndef VIDEOWINDOW_H
#define VIDEOWINDOW_H

#include <QWidget>
#include <QTimer>
#include <QLabel>
#include <opencv2/core/core.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include <opencv2/highgui/highgui.hpp>

class VideoWindow: public QWidget
{
   Q_OBJECT
   public:

       VideoWindow(QWidget* parent = nullptr): QWidget(parent), cap(nullptr) {};
       VideoWindow(char* adr, QWidget* parent = nullptr): QWidget(parent), cap(nullptr)
       {
           loadVideoCapture(adr);
       }

       void loadVideoCapture(char* adr)
       {
           cap = new cv::VideoCapture(adr);
           frameRate = (int) cap->get(cv::CAP_PROP_FPS);
           br = new QBrush();
           plt = new QPalette(this->palette());

           timer = new QTimer(this);
           connect(timer, SIGNAL(timeout()), this, SLOT(updatePicture()));
           timer->start(int(frameRate/10000));
       }

       ~VideoWindow()
       {
           cap->release();
           delete cap;
           image.release();
           delete br;
           delete plt;
       }


   public slots:
       void updatePicture()
       {
           *cap >> image;
           W = this->size().width();
           H = this->size().height();
           if (H < 10) H = 10;
           if (W < 10) W = 10;
           cv::resize(image, image, cv::Size(W, H), 0, 0, cv::INTER_LINEAR);
           cvtColor(image, image, cv::COLOR_BGR2RGB);
           image1 = QImage((uchar*) image.data, image.cols, image.rows, image.step, QImage::Format_RGB888);

           br->setTextureImage(image1);
           plt->setBrush(QPalette::Background, *br);
           this->setPalette(*plt);
       }

   private:
       int W, H;
       int frameRate;
       QTimer * timer;
       QBrush *br;
       QPalette *plt;
       QImage image1;
       cv::VideoCapture *cap;
       cv::Mat image;
};


#endif // VIDEOWINDOW_H

mainwindow.cpp:

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    QWidget* mainWidget = new QWidget( this );

    QGridLayout *mainLayout = new QGridLayout(mainWidget);

    A = new VideoWindow* [4];
    A[0] = new VideoWindow((char*)"rtsp://192.168.168.230:554/user=admin&password=&channel=1&stream=0:network-caching=0.sdp?", mainWidget);
    A[1] = new VideoWindow((char*)"rtsp://192.168.168.230:554/user=admin&password=&channel=2&stream=0:network-caching=0.sdp?", mainWidget);
    A[2] = new VideoWindow((char*)"rtsp://192.168.168.230:554/user=admin&password=&channel=3&stream=0:network-caching=0.sdp?", mainWidget);
    A[3] = new VideoWindow((char*)"rtsp://192.168.168.230:554/user=admin&password=&channel=4&stream=0:network-caching=0.sdp?", mainWidget);

    mainLayout->addWidget(A[0],0,0,1,1);
    mainLayout->addWidget(A[1],0,1,1,1);
    mainLayout->addWidget(A[2],1,0,1,1);
    mainLayout->addWidget(A[3],1,1,1,1);

    mainWidget->setLayout(mainLayout);
    setCentralWidget( mainWidget );
}

What am I doing wrong? Thanks in advance!
The VideoWindow.h file was created as a .h file. Perhaps the problem is that I did not use Qtr Designer?

[jsulm]

@Algorithm0 Hi and welcome!
You should call show() on your A[0..3] widgets.

[SGaist]

Hi,

You should also provide the code of your custom widget.

[Algorithm0]

@jsulm Hello! Sorry, but that doesn't work. I tried to insert a loop in different places, but the result remains the same. I also tried to display the main widget.

@SGaist Hello. So the VideoWindow.h file (VideoWindow class) is my custom widget. This is the first piece of code that I presented.