Why might there be a problem with asynchrony in this place?

Question

C++ / C#

Here is a code snippet:

private static Image Render(IPeakProvider peakProvider, WaveFormRendererSettings settings)
        {
            if (settings.DecibelScale)
                peakProvider = new DecibelPeakProvider(peakProvider, 48);

            var b = new Bitmap(settings.Width, settings.TopHeight + settings.BottomHeight);
            if (settings.BackgroundColor == Color.Transparent)
            {
                b.MakeTransparent();
            }
            using (var g = Graphics.FromImage(b))
            {
                g.FillRectangle(settings.BackgroundBrush, 0,0,b.Width,b.Height);
                var midPoint = settings.TopHeight;

                int x = 0;
                var currentPeak = peakProvider.GetNextPeak();
                while (x < settings.Width)
                {
                    var nextPeak = peakProvider.GetNextPeak();
                    
                    for (int n = 0; n < settings.PixelsPerPeak; n++)
                    {
                        var lineHeight = settings.TopHeight * currentPeak.Max;
                        g.DrawLine(settings.TopPeakPen, x, midPoint, x, midPoint - lineHeight);
                        lineHeight = settings.BottomHeight * currentPeak.Min;
                        g.DrawLine(settings.BottomPeakPen, x, midPoint, x, midPoint - lineHeight);
                        x++;
                    }
....

After I started calling the Render function asynchronously, via

...
var image = await Task.Run(() =>
{
return waveFormRenderer.Render(filePath, peakProvider, settings);
});
...

the program crashes with the error "Object is used elsewhere" on the line

g.DrawLine(settings.TopPeakPen, x, midPoint, x, midPoint - lineHeight);

I don't really understand how this can be, since the instance of g is created uniquely for each call to the Render function.