问关闭程序时，如何处理无法访问已释放的对象异常？

EN

private void Form1_FormClosing(object sender, FormClosingEventArgs e)
{

}

RichTextBoxExtensions.AppendText(rtbPaths, "Downloading: ", Color.Red);
RichTextBoxExtensions.AppendText(rtbPaths, downloader.CurrentFile.Path, Color.Green);
rtbPaths.AppendText(Environment.NewLine);

public class RichTextBoxExtensions
{
    public static void AppendText(RichTextBox box, string text, Color color)
    {
        box.SelectionStart = box.TextLength;
        box.SelectionLength = 0;

        box.SelectionColor = color;
        box.AppendText(text);
        box.SelectionColor = box.ForeColor;
    }

    public static void UpdateText(RichTextBox box, string find, string replace, Color? color)
    {
        box.SelectionStart = box.Find(find, RichTextBoxFinds.Reverse);
        box.SelectionLength = find.Length;
        box.SelectionColor = color ?? box.SelectionColor;
        box.SelectedText = replace;
    }
}

private void rtbPaths_TextChanged(object sender, EventArgs e)
{
    // set the current caret position to the end
    rtbPaths.SelectionStart = rtbPaths.Text.Length;
    // scroll it automatically
    rtbPaths.ScrollToCaret();
}