[REG] First row of QTreeWidget not user-checkable

#include <QApplication> 
#include <QTreeWidget> 

int main( int argc, char **argv ) 
{ 
QApplication app( argc, argv ); 

QTreeWidget *w = new QTreeWidget(); 

for (int i=0; i<4; i++) { 
QTreeWidgetItem *item = new QTreeWidgetItem(); 
item->setCheckState(0, Qt::Checked); 
w->addTopLevelItem(item); 
} 

w->show(); 
return app.exec(); 
}