我试图读取多个文件，以便可以从每个文件中提取信息。当我尝试读取第二个文件时，我得到

：未处理的异常： RangeError（索引）：无效值：仅有效值为0：1

我有一个名为file的对象，并且在初始化新文件之前尝试将其设置为null。

此功能来自错误：

    Future readFile() async {
      print("What text file would you like to read?(Don't forget the .txt)");
      var fileName = stdin.readLinesync();
      file=null;
      file = File(fileName);
    }

这是我叫的方法

  addExamGrade()async{
  await readFile();
  await file.readAsLines().then((lines) =>
      lines.forEach((l) => addIndividualEx(l))//print(l)
  );
}

我从这里的文件中读取信息：

addIndividualEx(String line)async{
  var split;
  var grade;
  String fname;
  String lname;
  double ExGrade;
  if (line != null) {
    split=line.split(',');
    grade=split[1].split(' ');
    lname=split[0];
    fname=grade[1];
    ExGrade=double.parse(grade[2]);
    if(!students.containsKey(fname)){//checks first name)
      students[fname]=Student(fname,lname);
    }
    students[fname].addExamGrade(ExGrade);
  }
}

我想读取多个文件。 但是相反，我得到了错误：

Unhandled exception:
RangeError (index): Invalid value: Only valid value is 0: 1
0      List.[] (dart:core-patch/growable_array.dart:147:60)
1      addIndividualHW (file:///C:/Users/Andrea%20Fernanda%20Torr/IdeaProjects/TestDart/bin/main.dart:117:16)
<asynchronous suspension>
2      addHomeworkGrades.<anonymous closure>.<anonymous closure> (file:///C:/Users/Andrea%20Fernanda%20Torr/IdeaProjects/TestDart/bin/main.dart:70:28)
3      List.forEach (dart:core-patch/growable_array.dart:283:8)
4      addHomeworkGrades.<anonymous closure> (file:///C:/Users/Andrea%20Fernanda%20Torr/IdeaProjects/TestDart/bin/main.dart:70:13)
5      _RootZone.runUnary (dart:async/zone.dart:1379:54)
6      _FutureListener.handleValue (dart:async/future_impl.dart:137:18)
7      Future._propagateToListeners.handleValueCallback (dart:async/future_impl.dart:678:45)
8      Future._propagateToListeners (dart:async/future_impl.dart:707:32)
9      Future._completeWithValue (dart:async/future_impl.dart:522:5)
10     Future._asyncComplete.<anonymous closure> (dart:async/future_impl.dart:552:7)
11     _microtaskLoop (dart:async/schedule_microtask.dart:41:21)
12     _startmicrotaskLoop (dart:async/schedule_microtask.dart:50:5)
13     _runPendingImmediateCallback (dart:isolate-patch/isolate_patch.dart:116:13)
14     _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:173:5)