How to add a widget to QMainWindow layout?

I'm using the code below to add a QPushButton to a ui:

    QPushButton *mine[PlayForm->horizontal][PlayForm->vertical];
    for(int i=0; i<PlayForm->horizontal; i++)
    {
        for(int j=0; j<PlayForm->vertical; j++)
        {
            QWidget wdg;
            mine[i][j] = new QPushButton(" ", &wdg);
            mine[i][j]->setGeometry(size*i, size*j, size, size);
            mine[i][j]->show();
            PlayForm->layout()->addWidget(mine[i][j]);
        }
    }

But I get this message:

QMainWindowLayout::addItem: Please use the public QMainWindow API instead

What should I do?

You are creating a temporary parent widget on the stack, and using a reference to it for the QPushButton - this is a fine way to get a segfault.

The parent widget argument defaults to nullptr, which is acceptable for you as the layout takes ownership of it.

//QWidget wdg;
mine[i][j] = new QPushButton( " " );

The error message you are getting is answered by this question. In short it means that you should not modify the QMainWindow layout; you need to a create a widget, add that as the central widget, and then modify the central widget's layout.