Qt QPainter paint on screen fails

[marcux]

Hi all!
I am programming in Qt and I am trying to draw a line on the screen, but it fails.
My header file:

#ifndef GAMEAREA_H
#define GAMEAREA_H

#include <QWidget>
#include <QPainter>
#include "character.h"

class GameArea : public QWidget
{
public:
    GameArea();
    QSize sizeHint() const;

protected:
    void paintEvent(QPaintEvent *event);

private:
    int leftStop;
    int rightStop;
    QPainter painter;
};

#endif // GAMEAREA_H

and my source file:

#include "gamearea.h"
#include <QPointF>
#include <QMessageBox>
#include <QDir>
#include <QPaintEvent>

GameArea::GameArea()
    : QWidget()
{
    leftStop = 20;
    rightStop = width() - 20;
    QPainter painter(this);
    setAutoFillBackground(true);
}

void GameArea::paintEvent(QPaintEvent *event)
{
    QPainterPath path;
    path.moveTo(10,10);
    path.lineTo(50, 10);
    path.lineTo(50, 50);
    painter.drawPath(path);
}

QSize GameArea::sizeHint() const
{
    return QSize(400, 300);
}

I add my drawing in a QMainWindow:

GameArea *gameArea = new GameArea();
QVBoxLayout *mainLayout = new QVBoxLayout;
mainLayout->addWidget(gameArea);
mainWidget->setLayout(mainLayout);
setCentralWidget(mainWidget);

The error I get is:
QPainter::begin: Paint device returned engine == 0, type: 1

As I understand from searching on the net I am trying to draw outside the drawing are.
By I can not understand why!!

Please help!!

Many thanks in advance!!

Marcux