setLayout: Attempting to set QLayout "" on MainWindow "MainWindow", which already has a layout

MainWindow::MainWindow(QWidget *parent)

: QMainWindow(parent)      

{      

QString xStr;      

xLabel = new QLabel(tr("x():"));      

xValueLabel = new QLabel;      

xValueLabel->setText(xStr.setNum(x()));      

QGridLayout *grdLayout = new QGridLayout;      

grdLayout->addWidget(xLabel,0,0);      

grdLayout->addWidget(xValueLabel,0,1);      

QWidget *widget = new QWidget( this);      

widget->setLayout( grdLayout) ;      

this->setCentralWidget(widget);      

}