flutter - 如何解决此NoSuchMethodError inFlutter问题？

Question

我正在关注在线类(class)，但是我的代码抛出了一个错误，而教练的代码却没有。是的，我已对其进行扫描以确保它们相同。
也许我错过了一些小细节。无论如何，有人可以帮助我解决此错误，以便我继续吗？
这是get方法:

  List<Map<String, Object>> get groupedTransactionValues {
    return List.generate(7, (index) {
      final weekDay = DateTime.now().subtract(
        Duration(days: index),
      );
  var totalSum = 0.0;

  for (int i; i < recentTransactions.length; i++) {
    if (recentTransactions[i].date.day == weekDay.day &&
        recentTransactions[i].date.month == weekDay.month &&
        recentTransactions[i].date.year == weekDay.year) {
      totalSum += recentTransactions[i].amount;
    }
  }

  return {
    'day': DateFormat.E().format(weekDay).substring(0, 1),
    'amount': totalSum,
  };
});

}
这是在Flutter中的实现:

  @override
  Widget build(BuildContext context) {
    return Card(
      elevation: 6,
      margin: EdgeInsets.all(20),
      child: Row(
        children: groupedTransactionValues.map((data) {

        }).toList(),
      ),
    );
  }
}

这是错误消息:

The following NoSuchMethodError was thrown building Chart(dirty):
The method '<' was called on null.
Receiver: null
Tried calling: <(0)

When the exception was thrown, this was the stack:
#0      Object.noSuchMethod  (dart:core-patch/object_patch.dart:51:5)
#1      Chart.groupedTransactionValues.<anonymous closure> 
package:expense/widgets/chart.dart:19
#2      new List.generate  (dart:core-patch/array_patch.dart:78:28)
#3      Chart.groupedTransactionValues 
package:expense/widgets/chart.dart:13
#4      Chart.build 
package:expense/widgets/chart.dart:45
#5      StatelessElement.build 
package:flutter/…/widgets/framework.dart:4620
#6      ComponentElement.performRebuild 
package:flutter/…/widgets/framework.dart:4546
#7      Element.rebuild 
package:flutter/…/widgets/framework.dart:4262
#8      ComponentElement._firstBuild 
package:flutter/…/widgets/framework.dart:4525
#9      ComponentElement.mount 
package:flutter/…/widgets/framework.dart:4520
#10     Element.inflateWidget 
package:flutter/…/widgets/framework.dart:3490
#11     MultiChildRenderObjectElement.mount 
package:flutter/…/widgets/framework.dart:5991
...     Normal element mounting (109 frames)
#120    Element.inflateWidget 
package:flutter/…/widgets/framework.dart:3490
#121    MultiChildRenderObjectElement.mount 
package:flutter/…/widgets/framework.dart:5991
...     Normal element mounting (210 frames)
#331    Element.inflateWidget 
package:flutter/…/widgets/framework.dart:3490
#332    MultiChildRenderObjectElement.mount 
package:flutter/…/widgets/framework.dart:5991
...     Normal element mounting (273 frames)
#605    Element.inflateWidget 
package:flutter/…/widgets/framework.dart:3490
#606    Element.updateChild 
package:flutter/…/widgets/framework.dart:3258
#607    RenderObjectToWidgetElement._rebuild 
package:flutter/…/widgets/binding.dart:1174
#608    RenderObjectToWidgetElement.mount 
package:flutter/…/widgets/binding.dart:1145
#609    RenderObjectToWidgetAdapter.attachToRenderTree.<anonymous closure> 
package:flutter/…/widgets/binding.dart:1087
#610    BuildOwner.buildScope 
package:flutter/…/widgets/framework.dart:2620
#611    RenderObjectToWidgetAdapter.attachToRenderTree 
package:flutter/…/widgets/binding.dart:1086
#612    WidgetsBinding.attachRootWidget 
package:flutter/…/widgets/binding.dart:927
#613    WidgetsBinding.scheduleAttachRootWidget.<anonymous closure> 
package:flutter/…/widgets/binding.dart:908
(elided 11 frames from class _RawReceivePortImpl, class _Timer, dart:async, and dart:async-patch)

Answer 1

for (int i; i < recentTransactions.length; i++) {

这是导致错误的部分，因为已声明:“方法'<'在null上被调用。”您必须初始化int i = 0，因此代码应如下所示

for (int i = 0; i < recentTransactions.length; i++) {