Why not enter the "try catch", but throw an exception externally? And who is null?

Firstly，the CAN device receives message,and calling the following method

private List<string> recordList = new List<string>();//
private List<string> recordList1 = new List<string>();//
private List<string> recordList2 = new List<string>();//

 public void FrameReceived(CANFrame frame)
 {

     if (!isRecord) return;
     if (frame != null)
     {
         string record = RecordFrame(frame, "Rx");
         recordList.Add(record);
         if (frame.Channel == 1) recordList1.Add(record);
         if (frame.Channel == 2) recordList2.Add(record);
     }
 }

 public string RecordFrame(CANFrame frame, string type)
 {
     string text1 = string.Empty;
     if (frame.type == TPCANMessageType.PCAN_MESSAGE_STANDARD)
     {
         text1 = ($"{stopwatch.ElapsedTicks * 1.0 / Stopwatch.Frequency,11:#0.000000} {frame.Channel}  {frame.Id,-15:X} {type}   d {frame.Length} {frame.ToHexString()}");
     }
     else if (frame.type == TPCANMessageType.PCAN_MESSAGE_EXTENDED)
     {
         text1 = ($"{stopwatch.ElapsedTicks * 1.0 / Stopwatch.Frequency,11:#0.000000} {frame.Channel}  " + $"{frame.Id:X}x".PadRight(15) + $" {type}   d {frame.Length} {frame.ToHexString()}");
     }
     else if (frame.type == TPCANMessageType.PCAN_MESSAGE_FD)
     {
         text1 = ($"{stopwatch.ElapsedTicks * 1.0 / Stopwatch.Frequency:#0.000000} CANFD {frame.Channel} {type} {frame.Id:X} 0 0 1 {frame.DLC:X2} {frame.Length} {frame.ToHexString()} 0 0 39b5 0 0 0 0");
     }
     
     else
     {
         if (frame.Data.Length <= 8)
         {
             text1 = ($"{stopwatch.ElapsedTicks * 1.0 / Stopwatch.Frequency,11:#0.000000} {frame.Channel}  " + (frame.Id > 0x7FF ? $"{frame.Id:X}x".PadRight(15) : $"{frame.Id,-15:X}") + $" {type}   d {frame.Length} {frame.ToHexString()}");
         }
         else
         {
             text1 = ($"{stopwatch.ElapsedTicks * 1.0 / Stopwatch.Frequency:#0.000000} CANFD {frame.Channel} {type} " + (frame.Id > 0x7FF ? $"{frame.Id:X}x" : $"{frame.Id:X}") + $" 0 0 1 {frame.DLC:X2} {frame.Length} {frame.ToHexString()} 0 0 39b5 0 0 0 0");
         }
     }
     return text1;

 }

Then,I call follwing method to store the message

 public void StartRecord(string fileName)
 {
     this.fileName = fileName;
     stopwatch.Reset();
     stopwatch.Start();

     recordList.Clear();
     recordList1.Clear();
     recordList2.Clear();
     isRecord = true;
 }
 public void StopRecord()
 {
     isRecord = false;
     WriteToFile();
     
     stopwatch.Stop();

 }

private async void WriteToFile()
{
    try
    {
        Task writeAll = WriteToFileAsync($"{fileName}\\合并CAN.asc", new List<string>(recordList));
        Task write1 = WriteToFileAsync($"{fileName}\\内CAN.asc", new List<string>(recordList1));
        Task write2 = WriteToFileAsync($"{fileName}\\外CAN.asc", new List<string>(recordList2));
        await Task.WhenAll(writeAll, write1, write2);
    }
    catch(Exception ex)
    {
        SmartFrameWork.Console.WriteLine("WriteToFile()");
        SmartFrameWork.Console.WriteLine(ex.ToString());
    }
}
async Task WriteToFileAsync(string fileName,List<string> lines)
{
    try
    {
        using (StreamWriter writer = new StreamWriter(fileName))
        {
            await writer.WriteLineAsync($"date {DateTime.Now.ToString("dd/M/yyyy HH:mm:ss.fff")}");
            await writer.WriteLineAsync("base hex  timestamps absolute");
            await writer.WriteLineAsync("no internal events logged");
            await writer.WriteLineAsync($"//Version 5.6");
            foreach (var line in lines)
            {
                if (writer == null)
                    SmartFrameWork.Console.WriteLine("writer为空");
                if (line == null)
                    SmartFrameWork.Console.WriteLine("写入该行的内容为空");
                await writer.WriteLineAsync(line);

            }
            await writer.FlushAsync(); 
        }
    }
    catch(Exception ex)
    {
        SmartFrameWork.Console.WriteLine("WriteToFileAsync(string fileName,List<string> lines)");
        SmartFrameWork.Console.WriteLine(ex.ToString());
    }
   
}

the exception infomation:

System.NullReferenceException：未将对象引I用设置到对象的实例。 在 System.IO.StreamWriter.<WriteAsynclnternal>d_55.MoveNext() ---引发异常的上一位置中堆栈跟踪的末尾· 在 System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) 在 System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) 在 System.Runtime.CompilerServices.TaskAwaiter.GetResulto 在 CANBusDriver.FileFrameFilter.<WriteToFileAsync>d_35.MoveNext() 位置 D:\Projects\HIL\HIL_ fanqingyu\V5.6Beta\HIL\CANdriver\FileFrameFilter.cs:行号157 --引发异常的上一位置中堆栈跟踪的末尾-- 在 System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) 在 System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) 在 System.Runtime.CompilerServices.TaskAwaiter.GetResult() 在 CANBusDriver.FileFrameFilter.<WriteToFile>d_34.MoveNext() 位置 D:\Projects\HIL\HIL_ fanqingyu\V5.6Beta\HIL\CANdriver\FileFrameFilter.cs:行号 145 --引发异常的上一位置中堆栈跟踪的末尾-- 在 System.Runtime.CompilerServices.AsyncMethodBuilderCore.<>c.≤ThrowAsync>b6_1(0bject state) 在 System.Threading.QueueUserWorkltemCallback.WaitCallback_ Context(Object state) 在 System.Threading.ExecutionContext.Runlnternal(ExecutionContext
executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx) 在 System.Threading.ExecutionContext.Run(ExecutionContext executionContext,ContextCallback callback, Object state, Boolean preserveSyncCtx) 在 System.Threading.QueueUserWorkitemCallback.System.Threading.IThreadPoolWorkltem.ExecuteWorkltem() 在 System.Threading.ThreadPoolWorkQueue.Dispatch() 在 System.Threading._ThreadPoolWaitCallback.PerformWaitCallback()

the line 157 means "await writer.WriteLineAsync(line);" the line 145 means "await Task.WhenAll(writeAll, write1, write2);" This exception is an occasional occurrence, not a definite occurrence,according to the exception information,there is a NullReferenceException，and the exception rised by the statement "await writer.WriteLineAsync(line);" .I am sure the "line" is not null,because before i add it to the List,its initial value is string.Empty,but the "writer" has worked.the "ChatGPT" tells me it happend inside the method of"await writer.WriteLineAsync(line);",i can not understand `