QCanvasPolygonalItem delete and SegFault or pure virtual call

[hayati]

ok, thanks for reply;
but i can't figure out what is wrong with my little tiny code. (and this code is absolutely written to simulate the error)
i don't mean find my errors; i mean why qt doesn't generate pure virtual call errors or segfault when i increase the Cell objects size (width or height) but if i decrease its width or height then it crushes. please check this out in provided source file, i stated the situation in comments.
so what is wrong with this code (that leads to this error) could anybody explain me?
moreover; i have nothing to do with QCanvas::update()
procedure is quite simple shrink(decrease the size of) the object, delete the object and QCanvas::update() func call and it crushes.
remember when you increase its size there is no problem! only when you decrease this happens.