QPainter

[marcel]

Because you don't instantiate the member QPainter.
You have two options:
1. Add:

painter = new QPainter();

painter = new QPainter();

To copy to clipboard, switch view to plain text mode 

in the constructor, and:

delete painter;

delete painter;

To copy to clipboard, switch view to plain text mode 

in the destructor.

2. Remove the member( recommended ) and in paint event:

QPainter painter(this);
painter.drawPolygon(polygoon);

QPainter painter(this);
painter.drawPolygon(polygoon);

To copy to clipboard, switch view to plain text mode 

Regards