flutter：同一变量在一个类中返回 null，但在另一个类中工作正常

Question

我有两个不同的类，1）

class PostCard extends StatefulWidget {
final Post? post;

const PostCard({Key? key, this.post}) : super(key: key);

@override
State<PostCard> createState() => _PostCardState();
}

第二类：

class History extends StatefulWidget {
static const id = '/history';
final Post? post;

const History({Key? key, this.post}) : super(key: key);

@override
_HistoryState createState() => _HistoryState();
}

在第一个类中 widget.post!.weight 返回它应该返回的值，但是在历史类中 widget.post!.weight 在“post!”上返回空错误。而且我不明白为什么？谢谢！ 这些值都包装在文本小部件中使用：

Text((int.parse(widget.post!.weight)* 
 (data.durationSeconds ~/ 
 60)).toStringAsFixed(2).padLeft(2, '0')),
                     

错误：

Null check operator used on a null value

 When the exception was thrown, this was the stack: 
 #0      _HistoryState.build.<anonymous closure> 
 (package:..../..../history.dart:319:56)
 #1      SliverChildBuilderDelegate.build 
 (package:flutter/src/widgets/sliver.dart:457:22)
 #2      SliverMultiBoxAdaptorElement._build 
 (package:flutter/src/widgets/sliver.dart:1215:28)
 #3      SliverMultiBoxAdaptorElement.performRebuild.processElement 
 (package:flutter/src/widgets/sliver.dart:1148:67)
 #4      Iterable.forEach (dart:core/iterable.dart:279:35)
 #5      SliverMultiBoxAdaptorElement.performRebuild 
 (package:flutter/src/widgets/sliver.dart:1192:24)
 #6      SliverMultiBoxAdaptorElement.update 
 (package:flutter/src/widgets/sliver.dart:1126:7)
  #7      Element.updateChild 
 (package:flutter/src/widgets/framework.dart:3370:15)