error in constructor

I want to program a class ProjectList, which is derived from class 
GtkTreeView. The following code snippet results in an assertion error. Is 
anybody able to explain me the error ?


class ProjectList extends GtkTreeView
{

    protected $store;

    /***********************************************************/
    /* Constructor Method
    /***********************************************************/
    function ProjectList()
    {
 
        $this->store = new GtkListStore(
		GObject::TYPE_STRING, GObject::TYPE_STRING
	);

        parent::__construct( $this->store );

                $col = 0;
                $column = $this->get_column(0);
                assert( $column );		// <- this line throws an assertion error
                $cell_renderer = new GtkCellRendererPixbuf(); 
                $column->pack_start($cell_renderer, false); 
                
		$column->set_cell_data_func(
			$cell_renderer, 
			"prjlist_format_pixbuf", 
			$this->Pixmap
		); 

    }

} // of class