Try using QVariant::setValue(..) for example:

// set
QWidget* widget = ui.MyWidget->window();

QVariant var;
var.setValue(widget);
listWidgetItem->setData(Qt::UserRole, var);

// get
QVariant var = item->data(Qt::UserRole);
QWidget* widget= var.value<QWidget*>();


I'm not completely sure, but so far I know you only can use this for classes inherited by QObject...

Montuno