I am working on a feature in my GUI that allow user to draw polygon by clicking the points.
First the user needs to enable this function by clicking a button. After that the user clicks any four points on the screen. When the fourth point is clicked, all the 4 points will be connected to form a polygon.

However the polygon is not appearing after the fourth is clicked. Instead there is a message of "QPainter::begin: Paint device returned engine == 0, type: 1". What is the problem in my code?

Qt Code: 
  1. bool QTGraphicsShape::eventFilter(QObject *obj, QEvent *event)
  2. {
  3.     double static x[4],y[4];
  4.     int static i;
  5.     if ((event->type() ==  QEvent::GraphicsSceneMouseRelease) && (Draw3Points == true)) {
  6.         QGraphicsSceneMouseEvent *mouseEvent = static_cast< QGraphicsSceneMouseEvent* >( event );
  7.         QPointF img_coord_pt = mouseEvent->scenePos();
  8.         x[i] = img_coord_pt.x();
  9.         y[i] = img_coord_pt.y();
  10.         i++;
  11.         if (i >= 4)
  12.         {
  13.             Draw3Points = false;
  14.             i=0;
  15.             static const QPointF points[4] = {
  16.                 QPointF(x[0], y[0]),
  17.                 QPointF(x[1], y[1]),
  18.                 QPointF(x[2], y[2]),
  19.                 QPointF(x[3], y[3])
  20.             };
  21.             QPainter painter(this);
  22.             painter.drawPolygon(points, 4);
  23.         }
  24.         return true;
  25.       } else {
  26.         return QObject::eventFilter(obj, event);
  27.       }
  28. }
  29.  
  30. void QTGraphicsShape::on_pushButton_clicked()
  31. {
  32.     ui.graphicsView->setMouseTracking(true);
  33.     Draw3Points = true;
  34.     m_pGraphicsScene->installEventFilter(this);
  35. }
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