I try to change the panelHeight and update the screen by setstate, but the StickIt widget seems to be corrupt as soon as I use setstate

Error after using after setstate: [ERROR:flutter/lib/ui/ui_dart_state.cc(199)] Unhandled Exception: Null check operator used on a null value E/flutter (18051): #0 _StickItState._exportImage (package:stick_it/src/stick_it.dart:192:56) E/flutter (18051): #1 StickIt.exportImage (package:stick_it/src/stick_it.dart:94:51)

code:

  StickIt _stickIt;
  bool showStickerBar=true;

  @override
  Widget build(BuildContext context) {
    
    _stickIt = StickIt(
      child: Image.file(widget.image, fit: BoxFit.cover),
      panelHeight:
      showStickerBar==true
          ? 200
          : 0,
      stickerList: [
        Image.asset('assets/stickers/test1.png'),
        Image.asset('assets/stickers/test2.png'),
      ],
    );

    return new Scaffold(
      appBar: PreferredSize(
        preferredSize: Size(double.infinity, 80),
        child: Stack(
          children: <Widget>[
                  InkWell(
                    onTap: () async {

                      final imageInUnit8List = await _stickIt.exportImage();
                      final Directory tempDir = await getTemporaryDirectory();
                      File imageWithSticker = await File('${tempDir.path}/image.png').create();
                      imageWithSticker.writeAsBytesSync(imageInUnit8List);

                    },
                    child: Text('next',
                      style: TextStyle(
                        fontSize: 17,
                      ),
                    ),
                  ),
                ],
              ),
            ),
      body: Center(
        child: Container(
          child: _stickIt,
        ),
      ),
      floatingActionButton:
            FloatingActionButton(
              onPressed: () {
                if (showStickerBar==true){
                  setState(() {
                    showStickerBar = false;
                  });
                }else if (showStickerBar==false){
                  setState(() {
                    showStickerBar = true;
                  });
                }
              },
              child: new Icon(
                Icons.emoji_emotions_outlined,
              ),
            ),

      

, I do not use null safety (unfortunately that is not possible)

thanks for help

is there a way to set the viewport to a fixed size, instead of taking all of the available space? When i use BoxFit.fitHeight the exported picture has a white area left and right, because stickit exports the entire screen width, but the image doesn't fill the complete width.

I tried it with: viewport: const Size(300, 600), But it has no effect.

My code:

     _stickIt = StickIt(
          viewport: const Size(300, 600),
          key: UniqueKey(),
          child: Image.file(widget.image!, 
              fit: BoxFit.fitHeight
          ),
          panelBackgroundColor: appColorBlack,
          panelHeight:200,
          stickerList: [
            Image.asset('assets/stickers/1.webp'),
            Image.asset('assets/stickers/2.webp'),
            Image.asset('assets/stickers/3.webp'),
          ],
        );

thanks

Add documentation to the package advanced example in regards to: why and when a Key is required.

• [ ] Adjust example.md
• [ ] Adjust advanced_example.dart

Reference issue: #1