qt 小程序 运行出异常

#include<QApplication>#include<QtGui>#include<QString>#include<QObject>int main(int argc,char *argv[]){    QApplication app(argc,argv);    QWidget *w=NULL;    QVBoxLayout *vb=NULL;    QLabel *l=NULL;//    QPushButton *pb=NULL;    QLineEdit *le=NULL;    QString pas("\0");    vb->addWidget(l);    vb->addWidget(le);    vb->addWidget(pb);    w->setLayout(vb);    l->setText(QObject::tr("QQ2012"));    le->setEchoMode(QLineEdit::Password);    pb->setText(QObject::tr("Login"));    pas=le->text();    w->show();    return app.exec();}