Returns the ID of the active row of @combo_box. This value is taken from the active row and the column specified by the GtkComboBox:id-column property of @combo_box (see gtk_combo_box_set_id_column()). The returned value is an interned string which means that you can compare the pointer by value to other interned strings and that you must not free it. If the GtkComboBox:id-column property of @combo_box is not set, or if no row is active, or if the active row has a %NULL ID value, then %NULL is returned.