CODE EXAMPLE FOR DART

scrolling to top sliverlist flutter with back button

// first add a scroll controller var
  final ScrollController _scrollController = ScrollController(
    initialScrollOffset: 0.0,
  );
  
// after add the code to set offset of controller at WillPopScope
return WillPopScope( onWillPop: () async {
      if (_scrollController.offset != 0.0) {
        _scrollController.animateTo(0.0,
            duration: const Duration(milliseconds: 250), curve: Curves.easeIn);
        return false;
      }else {
        return true;
      }
    },
   child: Scaffold( //...
   
// and finally set var controller to controller of your scroller like this:
CustomScrollView( controller: _scrollController, //...