how to disable ondismissed for listview.length = 1

im trying to make ondismmed for listview but i dont want to let user delete all children of the listview so when the listview has only 1 child the dismissble is disabled so how to do it???

onDismissed: (direction) {
     var item = containers.elementAt(index);
     if(containers.length > 1){
        deleteItem(index);
        Scaffold.of(context).showSnackBar(
           SnackBar(
               content: Text("Item deleted"),
               action: SnackBarAction(
               label: "UNDO",
               onPressed: () {
                    undoDeletion(index, item);
               }
          )
       )
  );
}
},
);