Exception: an object reference does not point to an instance of an object.

private static void button21_Click(object sender, EventArgs e)
{   
    string        line        = string.Empty;
    StringBuilder text        = new StringBuilder();
    List<string>  fileContent = new List<string>();
    int           lineCounter = 0;

    try
    {
        // "using" will initialize, and more importantly, finalize/cleanup 
        // file i/o automagically, allowing the programmer to concentrate 
        // on the actual purpose of the code. Notice there are no "Close"
        // statements, and more importantly, no "GOTO" statements. Another 
        // aspect is that this code is MUCH easier to read.

        // open the input file, and read the contents into a string list. This 
        // mitigates the possibility of throwing an exception during the read 
        // process, and also allws us to determine ahead of time whether we 
        // can ignore the operation because the file is empty.
        using (StreamReader sr = new StreamReader(@"c:\temp\Input.txt"))
        {
            // Read the entire file into a string list to avoid thrashing the 
            // hard drive. Change new line charachters ("\r\n") to a single 
            // character ("\n"), and then split the text on the single 
            // character.
            fileContent.AddRange(sr.ReadToEnd().Replace("\r\n", "\n").Split('\n'));
        }

        if (fileContent.Count > 0)
        {
            // your original code was difficult to read this early in the morning, 
            // but it appears as if you want to read lines from the source file until 
            // you've either read 6 lines, or until the last line read contains the 
            // word "End".

            // read the lines we want into a StringBuilder object. 
            do
            {
                line = fileContent[lineCounter];
                text.AppendLine(line);
                lineCounter++;
            } while ((lineCounter < fileContent.Count) && 
                     (lineCounter <= 5      || 
                      line.Contains(" End") || 
                      line.Trim().StartsWith("End")));

            using (TextWriter sw = File.CreateText(@"c:\temp\Output.txt"))
            {
                // now that we have our text, we can write all of it all at once.
                sw.Write(text.ToString());
            }
        }
    }
    catch (Exception ex)
    {
        // if a problem happens, we display the exception message, as well as the stack 
        // trace (the stack trace poitns to the code that failed).
        string error = string.Concat(ex.Message, 
                                     Environment.NewLine, 
                                     Environment.NewLine, 
                                     ex.StackTrace);
        //MessageBox.Show(error);
        Console.WriteLine(error);
    }
    // because the "using" statements above clean up for us, there's no reason to have 
    // a "finally" block. Also, all of the objects go outr of scope, so the memory they 
    // consumed is freed up.
}