How to retrieve a QWidget from a list of opened windows

In some part of my project, I want to get the reference of a window, from a list of opened windows. So, i'm doing this way:

QWidget* WindowUtil::mainWindow() {
    QWidget* main_window = nullptr;
    for(QWidget *window: QApplication::allWidgets()){
        if(QString(window>metaObject()->className()).contains("Home")){
            main_window = window;
            break;
        }
    }
    return main_window;
}

WindowUtil is class of my project and mainWindow() is a static method.

However, this solution doesn't work. The compiler says:

error: incomplete type 'QApplication' used in nested name specifier
 for(QWidget *window : QApplication::allWidgets()){
                       ^

And I'm stuck here.