Adding custom widget ( class ) into made in Qt Designer ui class

[freely]

Hello.
I have a custom widget class ( inherited from QWidget ) and an UI class made in Qt Designer and attached to mainwindow widget by ..->setupUi(…) method.
In the UI class I’ve put QWidget in Qt Designer ( empty region ) and I need to put inside it my custom widget class ( inherited from QWidget ).
I tried to make this by setting parent when construction custom widget class:

...
    MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    mc(new Ui::MainControl),intern(new Ui::Internal)
{
    mc->setupUi(this);
    intern->setupUi(this->mc->centralwidget);
    //constructing image viewing:
    image_preview=new ImageView();
    image_preview->setParent(intern->widget_image);
...

...
    MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    mc(new Ui::MainControl),intern(new Ui::Internal)
{
    mc->setupUi(this);
    intern->setupUi(this->mc->centralwidget);
    //constructing image viewing:
    image_preview=new ImageView();
    image_preview->setParent(intern->widget_image);
...

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But I get segmentation fault on Linux when I run the program. When I comment the last row:

//image_preview->setParent(intern->widget_image);

//image_preview->setParent(intern->widget_image);

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There is no seg. fault but image_preview is not added to intern UI class.
Does anybody know how to add the custom widget image_preview to the UI class member intern->widget_image , so the widget will be seen and active on the UI?
Thanks a lot for help.