错误：在 null 上调用了方法“round”。 接收方：null 尝试调用：round()。 构建小部件时抛出错误

Question

In my program, I'm pulling values from cloud firestore as an initial value and want them to react on user input in case of the slider. However, the aforementioned error gets thrown. I haven't posted the entire class as it didn't let me. Just kept it to the essentials. Do let me know if you require further information. Thanks in advance!

Here's my code:

class _ListPlusScreenState extends State<ListPlusScreen> {
  @override
  double _progress;
  
  String _name;

  Widget _BuildListItem(BuildContext context, DocumentSnapshot doc) {
    _progress = doc['Progress'];
    final _name = doc['Name'].toString();
    final _due = doc['Due'];
    final _priority = doc['Priority'];
    DocumentReference _listRef =
        Firestore.instance.collection("List Items").document();

    return Container(
      padding: EdgeInsets.all(15),
      width: 200,
      height: 600,
      decoration: BoxDecoration(
        borderRadius: BorderRadius.circular(20),
        color: Color(0xffacb3b8),
      ),
      child: Column(
        crossAxisAlignment: CrossAxisAlignment.start,
        children: [
          Row(
            children: [
              Text(
                _name,
                style: TextStyle(fontSize: 20),
              ),
              Checkbox(value: false, onChanged: null)
            ],
          ),
          Slider(
            value: _progress,
            min: 0.0,
            max: 100.0,
            divisions: 10,
            label: _progress.round().toString(),
            onChanged: (value) {
              setState(
                () {
                  _progress = value;
                  _listRef.updateData({'Progress': value});
                },
              );
            },
          ),
        ],
      ),
    );
  }

  Widget build(BuildContext context) {
    return Container(
      padding: EdgeInsets.all(10),
      decoration: BoxDecoration(
        color: const Color(0xFF6EC6CA),
        shape: BoxShape.rectangle,
        borderRadius: BorderRadius.circular(20.0),
      ),
      child: StreamBuilder(
          stream: Firestore.instance.collection('List Items').snapshots(),
          builder: (context, snapshot) {
            if (!snapshot.hasData) return const Text('Loading..');
            return ListView.builder(
              itemExtent: 150,
              itemCount: snapshot.data.documents.length,
              itemBuilder: (context, index) =>
                  _BuildListItem(context, snapshot.data.documents[index]),
            );
          }),
    );
  }
}

            ```

Answer 1

double _progress =0.0;
label: _progress?.round().toString()??"0.0",

Change two lines of code in your code