WindowCreationData::create: CreateWindowEx failed (Cannot create a top-level child window.)
ASSERT: "platformWindow" in file kernel\qwindow.cpp, line 396

[The ViewWidget is recreated with parentHandle=0x0 and WS_CHILD]

#include <QMainWindow>
#include <QDialog>
#include <QPushButton>
#include <QHBoxLayout>
#include <QApplication>

// ViewWidget
class ViewWidget : public QWidget
{
public:
    ViewWidget(QWidget *parent = nullptr) : QWidget(parent)
    {
        winId(); // grab the native window handle to pass to external lib responsible for painting the view...
    }
};

// ViewDialog
class ViewDialog : public QDialog
{
public:
    ViewDialog(QWidget *parent = nullptr) : QDialog(parent)
    {
        setWindowTitle(tr("View Dialog"));
        resize(160, 90);

        ViewWidget* view = new ViewWidget;

        QHBoxLayout* layout = new QHBoxLayout;
        layout->addWidget(view);

        setLayout(layout);
    }
};

// MainWindow
class MainWindow : public QMainWindow
{
    //Q_OBJECT

public:
    MainWindow(QWidget *parent = nullptr) : QMainWindow(parent)
    {
        setWindowTitle(tr("Qt 5.6.0 beta bug Test"));
        resize(240, 120);

        QPushButton* button = new QPushButton("Click me!", this);
        connect(button, &QPushButton::clicked, this, &MainWindow::openDialog);
        setCentralWidget(button);
    }

    ~MainWindow() {}

public slots:
    void openDialog()
    {
        static ViewDialog* dlg = nullptr;

        if (!dlg) dlg = new ViewDialog;

        dlg->show();
    }
};

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    MainWindow main;
    main.show();

    return app.exec();
}