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