flutter_inner_drawer error: type 'List<Widget?>' is not a subtype of type 'List<Widget>' in type cast

Question

Using flutter_inner_drawer: ^1.0.0+1 I got a error thats is type

'List<Widget?>' is not a subtype of type 'List' in type cast

my code:

   final GlobalKey<InnerDrawerState> _innerDrawerKey =
     GlobalKey<InnerDrawerState>();

   InnerDrawer(
    key: _innerDrawerKey,
    onTapClose: true, // default false
    swipe: true, // default true
    colorTransitionChild: Colors.red, // default Color.black54
    colorTransitionScaffold: Colors.black54, // default Color.black54

    //When setting the vertical offset, be sure to use only top or bottom
    offset: const IDOffset.only(bottom: 0.05, right: 0.0, left: 0.0),
    scale:
        const IDOffset.horizontal(0.8), // set the offset in both directions

    proportionalChildArea: true, 
    borderRadius: 50, // default 0
    leftAnimationType: InnerDrawerAnimation.static, 
    rightAnimationType: InnerDrawerAnimation.quadratic,
    backgroundDecoration: const BoxDecoration(
        color: Colors.red), /

    onDragUpdate: (double? val, InnerDrawerDirection? direction) {
     
      print(val);
    
      print(direction == InnerDrawerDirection.start);
    },
    innerDrawerCallback: (a) =>
        print(a), 
    leftChild: Container(), 
    
    scaffold: Scaffold(
      appBar: AppBar(automaticallyImplyLeading: false),
    )
  
    );

how can I salve this error?

Answer 1

This issue is coming from the source code. You can check the git Issue

The issue is coming from L435-L437

    final Widget scaffoldChild = Stack(
      children: <Widget?>[widget.scaffold, invC != null ? invC : null]
          .where((a) => a != null)
          .toList() as List<Widget>,
    );

children doesn't accept any nullable widget.

Also, _trigger(..) method returning nullable widget L544-L563

And It is being used on L600-L620

While the children can't be null anymore, we can remove or replace _trigger() widget when it returns null.

You can solve by making children not nullable.

In that case scaffoldChild will be

 final Widget scaffoldChild = Stack(
      children: <Widget>[
        widget.scaffold,
        if (invC != null) invC,
      ].toList(),
    );

null check while using _trigger(..) or provide default value like

_trigger(AlignmentDirectional.centerStart, _leftChild) ??
                      SizedBox(),
_trigger(AlignmentDirectional.centerEnd, _rightChild) ??
                      SizedBox(),
                ].toList(),

Make sure to replace <Widget?>[ with <Widget>[

There is an easy solution from others repo: you can follow this comment which says

Hi, i used a pull request. In the pubspec.yaml, i change for this:

flutter_inner_drawer: 
 git: 
   url: https://github.com/kenjishiromajp/flutter_inner_drawer 
   ref: kenjishiromajp-update-for-2.12-null-safety-casting