How to solve and avoid from getting this exception ?

in form1 i have this button click event code

private async void buttonAnalyze_Click(object sender, EventArgs e)
{
    if (_pointsOnImage.Count > 0 && _segmentAnalyzer != null)
    {
        string outputFilePath = Path.Combine(jsonFilesFolder, "analysis.csv");
        var progress = new Progress<int>(frameNumber =>
        {
            trackBarFrames.Value = frameNumber;
            labelFrames.Text = $"Analyzing frame {frameNumber + 1}/{_totalFrames}";
        });

        await _segmentAnalyzer.GenerateCsvFileAsync(_totalFrames, _videoCapture, outputFilePath, progress);
        MessageBox.Show("Analysis complete. CSV file saved.");
    }
    else
    {
        if (_pointsOnImage.Count == 0)
        {
            MessageBox.Show("_pointsOnImage list contains no elements.");
        }

        if (_segmentAnalyzer == null)
        {
            MessageBox.Show("Make sure that _segmentAnalyzer is not null.");
        }
    }
}

and in the class SegmentAnalyzer i have this method code

public async Task GenerateCsvFileAsync(int totalFrames, VideoCapture videoCapture, string outputFilePath, IProgress<int> progress)
{
    int squareSize = CalculateSquareSize();
    using (StreamWriter writer = new StreamWriter(outputFilePath))
    {
        await writer.WriteLineAsync("FRAME_NUMBER,MINUS,LEFTDIGIT_TOP,LEFTDIGIT_LEFTTOP,...,RIGHTDIGIT_RIGHTBOTTOM,RIGHTDIGIT_BOTTOM");

        for (int frameNumber = 0; frameNumber < totalFrames; frameNumber++)
        {
            videoCapture.Set(VideoCaptureProperties.PosFrames, frameNumber);
            Mat frame = new Mat();
            if (videoCapture.Read(frame))
            {
                Bitmap bitmap = BitmapConverter.ToBitmap(frame);
                bmp = bitmap;
                string line = $"{frameNumber}";

                foreach (var point in _pointsOnImage)
                {
                    Rectangle square = new Rectangle(point.X - squareSize / 2, point.Y - squareSize / 2, squareSize, squareSize);
                    double grayLevelAverage = CalculateGrayLevelAverage(square);
                    line += $",{grayLevelAverage}";
                }

                await writer.WriteLineAsync(line);
            }
            progress?.Report(frameNumber);
        }
    }
}

the exception is on the line:

using (StreamWriter writer = new StreamWriter(outputFilePath))

System.IO.IOException: ‘The process cannot access the file ‘d:savedpointsanalysis.csv’ because it is being used by another process.’