Null Check operator used on a null value - flutter

Question

when click the showModalBottomSheet it's show on the screen
Null Check operator used on a null value
can anyone help to figure out which is the code error? kindly refer full code as below, TQ!
class ProductContentFirst extends StatefulWidget {
final List _productContentList;
const ProductContentFirst(this._productContentList, {Key? key})
: super(key: key);
#override
State<ProductContentFirst> createState() => _ProductContentFirstState();
}
class _ProductContentFirstState extends State<ProductContentFirst> {
ProductContentItem? _productContent;
List _attr = [];
#override
void initState() {
super.initState();
_productContent = widget._productContentList[0];
this._attr = this._productContent!.attr!;
//print(this._attr);
}
List<Widget>? _getAttrItemWidget(attrItem) {
List<Widget> attrItemList = [];
attrItem.list.forEach((item) {
attrItemList.add(Container(
margin: EdgeInsets.all(5),
child: Chip(
label: Text("${item}"),
padding: EdgeInsets.all(10),
),
));
print (item);
});
}
List<Widget>? _getAttrWidget() {
List<Widget> attrList = [];
this._attr.forEach((attrItem) {
attrList.add(Wrap(
children: [
Container(
width: ScreenAdapter.width(80.0),
child: Padding(
padding: EdgeInsets.only(top: ScreenAdapter.height(30.0)),
child: Text(
"${attrItem.cate}",
style: TextStyle(
fontWeight: FontWeight.bold,
),
),
),
),
Container(
width: ScreenAdapter.width(600.0),
child: Wrap(
//children: [],
children: _getAttrItemWidget(attrItem)!,
),
)
],
));
print(attrItem.cate);
});
//return null;
}
_attrBottomSheet() {
showModalBottomSheet(
context: context,
builder: (context) {
return GestureDetector(
onTap: () {
false;
},
child: Stack(
children: [
Container(
padding: EdgeInsets.all(10),
child: ListView(
children: [
Column(
children: _getAttrWidget()!,
)
],
),
),
Positioned(
bottom: 0,
width: ScreenAdapter.width(750.0),
height: ScreenAdapter.height(100),
child: Row(
children: [
Expanded(
flex: 1,
child: Container(
child: JdButton(
color: Color.fromRGBO(253, 1, 0, 0.9),
text: "Add Cart",
cb: () {
print("Add Cart");
},
),
)),
Expanded(
flex: 1,
child: Container(
child: JdButton(
color: Color.fromRGBO(255, 165, 0, 0.9),
text: "Buy",
cb: () {
print("Buy");
},
),
)),
],
))
],
),
);
});
}
#override
Widget build(BuildContext context) {
String pic = Config.domain + this._productContent!.pic!;
pic = pic.replaceAll("\\", "/");
return Container(
padding: EdgeInsets.all(10.0),
child: ListView(
children: [
AspectRatio(
aspectRatio: 16 / 12,
child: Image.network(
"${pic}",
fit: BoxFit.cover,
),
),
Container(
padding: EdgeInsets.only(top: 7),
child: Text(
"${this._productContent!.title}",
style: TextStyle(
color: Colors.black87, fontSize: ScreenAdapter.size(36)),
),
),
Container(
padding: EdgeInsets.only(top: 7),
child: Text(
"${this._productContent!.subTitle}",
style: TextStyle(
color: Colors.black54, fontSize: ScreenAdapter.size(28)),
),
),
Container(
padding: EdgeInsets.only(top: 7),
child: Row(
children: [
Expanded(
flex: 1,
child: Row(
children: [
Text("Price: "),
Text(
"${this._productContent!.price}",
style: TextStyle(
color: Colors.red,
fontSize: ScreenAdapter.size(46)),
)
],
),
),
Expanded(
flex: 1,
child: Row(
mainAxisAlignment: MainAxisAlignment.end,
children: [
Text("Old Price: "),
Text(
"${this._productContent!.oldPrice}",
style: TextStyle(
color: Colors.black38,
fontSize: ScreenAdapter.size(32),
decoration: TextDecoration.lineThrough),
),
],
),
),
],
),
),
Container(
margin: EdgeInsets.only(top: 7),
height: ScreenAdapter.height(80.0),
child: InkWell(
onTap: () {
_attrBottomSheet();
},
child: Row(
children: [
Text(
"Select：",
style: TextStyle(fontWeight: FontWeight.bold),
),
Text("115, Black, XL, 1 pcs")
],
),
),
),
Divider(),
Container(
height: ScreenAdapter.height(80.0),
child: Row(
children: [
Text(
"Delivery Fees：",
style: TextStyle(fontWeight: FontWeight.bold),
),
Text("Free Delivery")
],
),
),
Divider(),
],
),
);
}
}
class ProductContentModel {
ProductContentItem? result;
ProductContentModel({this.result});
ProductContentModel.fromJson(Map<String, dynamic> json) {
result = json['result'] != null
? new ProductContentItem.fromJson(json['result'])
: null;
}
Map<String, dynamic> toJson() {
final Map<String, dynamic> data = new Map<String, dynamic>();
if (this.result != null) {
data['result'] = this.result!.toJson();
}
return data;
}
}
class ProductContentItem {
String? sId;
String? title;
String? cid;
Object? price;
String? oldPrice;
Object? isBest;
Object? isHot;
Object? isNew;
String? status;
String? pic;
String? content;
String? cname;
List<Attr>? attr;
String? subTitle;
Object? salecount;
ProductContentItem(
{this.sId,
this.title,
this.cid,
this.price,
this.oldPrice,
this.isBest,
this.isHot,
this.isNew,
this.status,
this.pic,
this.content,
this.cname,
this.attr,
this.subTitle,
this.salecount});
ProductContentItem.fromJson(Map<String, dynamic> json) {
sId = json['_id'];
title = json['title'];
cid = json['cid'];
price = json['price'];
oldPrice = json['old_price'];
isBest = json['is_best'];
isHot = json['is_hot'];
isNew = json['is_new'];
status = json['status'];
pic = json['pic'];
content = json['content'];
cname = json['cname'];
if (json['attr'] != null) {
attr = <Attr>[];
json['attr'].forEach((v) {
attr!.add(new Attr.fromJson(v));
});
}
subTitle = json['sub_title'];
salecount = json['salecount'];
}
Map<String, dynamic> toJson() {
final Map<String, dynamic> data = new Map<String, dynamic>();
data['_id'] = this.sId;
data['title'] = this.title;
data['cid'] = this.cid;
data['price'] = this.price;
data['old_price'] = this.oldPrice;
data['is_best'] = this.isBest;
data['is_hot'] = this.isHot;
data['is_new'] = this.isNew;
data['status'] = this.status;
data['pic'] = this.pic;
data['content'] = this.content;
data['cname'] = this.cname;
if (this.attr != null) {
data['attr'] = this.attr!.map((v) => v.toJson()).toList();
}
data['sub_title'] = this.subTitle;
data['salecount'] = this.salecount;
return data;
}
}
class Attr {
String? cate;
List<String>? list;
Attr({this.cate, this.list});
Attr.fromJson(Map<String, dynamic> json) {
cate = json['cate'];
list = json['list'].cast<String>();
}
Map<String, dynamic> toJson() {
final Map<String, dynamic> data = new Map<String, dynamic>();
data['cate'] = this.cate;
data['list'] = this.list;
return data;
}
}
When the exception was thrown, this was the stack:
\#0 \_ProductContentFirstState.\_getAttrWidget.\<anonymous closure (package:flutter_jdshop/pages/ProductContent/ProductContentFirst.dart:64:54)

Answer 1

Don't use ! without checking null. While the snippet is large, follow these steps.
Check null, and then perform operation the place you've used !. It will be like if(result!=null)result.add(new FocusItemModel.fromJson(v));
the children can be children: _getAttrItemWidget(attrItem)??[]
You aren't returning widgets from _getAttrItemWidget and others. It will be
List<Widget> _getAttrItemWidget(attrItem) { // no need to return null
List<Widget> attrItemList = [];
attrItem.list.forEach((item) {
....
print (item);
});
return attrItemList;
}
In short Don't use ! without checking null. or provide default value on null case.
Find more about null-safety.