How to add scrollbar...?

[jojanpm]

How can v add a scrollbar to a listWidget-checkBox combination? ('combination' means thr ll be one checkbox corresponding to (or on right of) each listWidget item)

i tried this:
created a listWidget and a set of, say 10, checkBox ctrls.
then created a widget, and dragged all into the widget.
then created a scrollArea, and dragged the widget into the scrollArea. (this step may not be necessary)
then added this code on pageload, as per the tutorial says:
ui->scrollArea->setWidget(ui->widget);

but i get runtime error. (no helpful info shown about the error)
WHY!?