Ошибка выполнения в построителе строк, который был объявлен статическим

Мой код для вставки данных выглядит следующим образом: он работает впервые, если я пытался вставить данные во второй раз, я получаю сообщение об ошибке

Мой полный код

  using System;
using System.Collections.Generic;
using System.Text;
using System.IO;
namespace ACHDAL
{
public class EntryDetail
{

    int[] debits ={ 25, 26, 27, 28, 29, 35, 36, 37, 38, 39, 46, 47, 48, 48, 49, 55, 56, 81, 84, 86, 88 };
    int[] credits ={ 20, 21, 22, 23, 24, 30, 31, 32, 33, 34, 40, 41, 42, 43, 44, 45, 50, 51, 52, 53, 54, 82, 83, 85, 87 };
    private string m_strRecordTypeCode;
    private string m_strTransactionCode;
    private string m_strRecievingDFIIdentification;
    private string m_strCheckDigit;
    private string m_strDFIAccountNumber;
    private string m_strAmount;
    private string m_strIdentificationNumber;
    private string m_strRecievingcompanyName;
    private string m_strDiscretionaryData;
    private string m_strAddendaRecordIndicator;
    private string m_strTraceNumber;
    private static string m_path = string.Empty;

    private bool m_flag = false;

    public string RecordTypeCode
    {
        get { return m_strRecordTypeCode; }
        set { m_strRecordTypeCode = value; }
    }

    public string TransactioCode
    {
        get { return m_strTransactionCode; }
        set { m_strTransactionCode = value; }
    }

    public string RecievingDFIIdentification
    {
        get { return m_strRecievingDFIIdentification; }
        set { m_strRecievingDFIIdentification = value; }
    }

    public string CheckDigit
    {
        get { return m_strCheckDigit; }
        set { m_strCheckDigit = value; }
    }

    public string DFIAccountNumber
    {

        get { return m_strDFIAccountNumber; }
        set { m_strDFIAccountNumber = value; }
    }

    public string Amount
    {
        get { return m_strAmount; }
        set { m_strAmount = value; }
    }

    public string IdentificationNumber
    {
        get { return m_strIdentificationNumber; }
        set { m_strIdentificationNumber = value; }
    }

    public string RecievingCompanyName
    {
        get { return m_strRecievingcompanyName; }
        set { m_strRecievingcompanyName = value; }
    }

    public string DiscretionaryData
    {
        get { return m_strDiscretionaryData; }
        set { m_strDiscretionaryData = value; }
    }

    public string AddendaRecordIndicator
    {
        get { return m_strAddendaRecordIndicator; }
        set { m_strAddendaRecordIndicator = value; }
    }

    public string TraceNumber
    {
        get { return m_strTraceNumber; }
        set { m_strTraceNumber = value; }
    }

    public bool addEntrydetails()
    {
        this.m_flag = false;
        if (m_strRecordTypeCode != string.Empty && m_strTransactionCode != string.Empty && m_strRecievingDFIIdentification != string.Empty &&
            m_strCheckDigit != string.Empty && m_strDFIAccountNumber != string.Empty && m_strAmount != string.Empty && m_strRecievingcompanyName != string.Empty
            && m_strAddendaRecordIndicator != string.Empty)
        {
            saveEntry(m_path);
            m_flag = true;
        }
        else
        {
            m_flag = false;
        }
        return m_flag;
    }

    public bool BankRoutingNumberValidation(string sInput)
    {
        m_flag = false;

        bool bRule1 = false;
        bool bRule2 = false;

        int iResult = 0;
        if (sInput.Length == 9)
        {
            iResult = 3 * Convert.ToInt32(sInput.Substring(0, 1)) +
            7 * Convert.ToInt32(sInput.Substring(1, 1)) +
            Convert.ToInt32(sInput.Substring(2, 1)) +
            3 * Convert.ToInt32(sInput.Substring(3, 1)) +
            7 * Convert.ToInt32(sInput.Substring(4, 1)) +
            Convert.ToInt32(sInput.Substring(5, 1)) +
            3 * Convert.ToInt32(sInput.Substring(6, 1)) +
            7 * Convert.ToInt32(sInput.Substring(7, 1)) +
            Convert.ToInt32(sInput.Substring(8, 1));

            if ((iResult % 10) == 0)
            {
                bRule1 = true;

                iResult = 7 * Convert.ToInt32(sInput.Substring(0, 1)) +
                3 * Convert.ToInt32(sInput.Substring(1, 1)) +
                9 * Convert.ToInt32(sInput.Substring(2, 1)) +
                7 * Convert.ToInt32(sInput.Substring(3, 1)) +
                3 * Convert.ToInt32(sInput.Substring(4, 1)) +
                9 * Convert.ToInt32(sInput.Substring(5, 1)) +
                7 * Convert.ToInt32(sInput.Substring(6, 1)) +
                3 * Convert.ToInt32(sInput.Substring(7, 1));

                if ((iResult % 10) == Convert.ToInt32(sInput.Substring(8, 1)))
                {
                    bRule2 = true;
                }

                m_flag = bRule1 & bRule2;
            }
        }
        return m_flag;

    }


    public bool saveEntry(string strPath)
    {
        long Amnt = 0;
        double AmtforFileControl_debit=0;
        double AmtforFileControl_credit=0;
        m_flag = true;

        Append.sb.AppendLine(); 
             //When i put a break point i am getting the error here               Object reference not set to an instance of an object.


        Append.sb.Append(m_strRecordTypeCode.PadLeft(1, '0'));
        Append.sb.Append(m_strTransactionCode.PadLeft(2, '0'));

        Append.sb.Append(m_strRecievingDFIIdentification.PadLeft(9, '0'));
        Append.sb.Append(m_strDFIAccountNumber.PadRight(17, ' '));
        Append.sb.Append(m_strAmount.PadLeft(10, '0'));
        int length = debits.Length;
        for (int j = 0; j < length; j++)
        {
            if (m_strTransactionCode == debits[j].ToString())
            {
                Amnt = Convert.ToInt64(m_strAmount);
                if(Append.oldbatchcontrol!=string.Empty)
                {
                    AmtforFileControl_debit=Convert.ToDouble(m_strAmount);
                }
                Append.debitAmnt += Amnt;
                break;
            }
        }
        int CreditLength = credits.Length;
        for (int k = 0; k < CreditLength; k++)
        {
            if (m_strTransactionCode == credits[k].ToString())
            {
                Amnt = Convert.ToInt64(m_strAmount);
                if(Append.oldbatchcontrol!=string.Empty)
                {
                    AmtforFileControl_credit=Convert.ToDouble(m_strAmount);
                }
                Append.creditAmnt += Amnt;
                break;
            }
        }
        Append.sb.Append(m_strIdentificationNumber.PadRight(15, ' '));
        Append.EntryHash += Convert.ToDouble(m_strRecievingDFIIdentification);
        Append.sb.Append(m_strRecievingcompanyName.PadRight(22, ' '));
        Append.sb.Append(m_strDiscretionaryData.PadRight(2, ' '));
        Append.sb.Append(m_strAddendaRecordIndicator.PadLeft(1, '0'));
        Append.sb.Append(m_strTraceNumber.PadLeft(15,'0'));

        //Entry Count for EntryDetail
        Append.Entrycnt++;

        if (Append.oldbatchcontrol != string.Empty)
        {
            //Append.sb.AppendLine();
            string strRecordtype = "8";

            string strServcclasscode = Append.StandEntryCode.PadLeft(3, '0');
            //m_strServiceClassCode.PadLeft(3, '0').ToString();

            string Entrycnt = Append.Entrycnt.ToString().PadLeft(6, '0');

            filecontrolvariables.entrycount++;

            string EntryHash = Append.EntryHash.ToString().PadLeft(10, '0');

            filecontrolvariables.Entryhash += Convert.ToDouble(m_strRecievingDFIIdentification);

            string debitAmnt = Append.debitAmnt.ToString().PadLeft(12, '0');

            filecontrolvariables.debitamt += AmtforFileControl_debit;

            string creditAmnt = Append.creditAmnt.ToString().PadLeft(12, '0');

            filecontrolvariables.creditamt = AmtforFileControl_credit;

            string CompIdentification = Append.Companyidentification.PadRight(10, ' ');
            //m_strCompanyIdentification.PadRight(10, ' ');

            string MessageAuthenticationCode = "".PadRight(19, (char)32).ToString();

            string Reserved = "".PadRight(6, (char)32).ToString();

            string Odfi = Append.OriginalOdfi.PadLeft(8, '0');
            //m_strOriginationDFIIdentification.PadLeft(8, '0');

            string batchno = Append.Batchnum_edit.ToString().PadLeft(7, '0');
            //m_strBatchNumber.PadLeft(7, '0');

            Append.sb.AppendLine();
            Append.sb.Append(strRecordtype);
            Append.sb.Append(strServcclasscode);
            Append.sb.Append(Entrycnt);
            Append.sb.Append(EntryHash);
            Append.sb.Append(debitAmnt);
            Append.sb.Append(creditAmnt);
            Append.sb.Append(CompIdentification);
            Append.sb.Append(MessageAuthenticationCode);
            Append.sb.Append(Reserved);
            Append.sb.Append(Odfi);
            Append.sb.Append(batchno);

            StreamReader forAddEntry = new StreamReader(strPath);
            string Filedata = string.Empty;
            while (Filedata == forAddEntry.ReadToEnd())
            {
                Filedata.Replace(Append.oldbatchcontrol, Append.sb.ToString());
            }
            forAddEntry.Close();
            StreamWriter sw = new StreamWriter(strPath);
            sw.Write(Filedata);
            sw.Close();
        }

        //Append.Batchcnt++;

        return m_flag;
    }
}
public static class Append
{
    public static string OriginalOdfi=string.Empty;
    public static string oldbatchcontrol=string.Empty;
    public static StringBuilder sb = new StringBuilder();
    public static StringBuilder _sb
    {
        get { return sb; }
        set
        {
            sb = value;
        }
    }

    public static int Batchcnt = 0;
    public static int Batchnum_edit=0;
    public static int Traceno = 0;
    public static int Entrycnt = 0;
    public static double EntryHash = 0;
    public static long debitAmnt = 0;
    public static long creditAmnt = 0;
    public static int Seccode = 0;
    public static string Companyidentification = string.Empty;
    public static string StandEntryCode = string.Empty;

}

}

Stack Trace

         at ACHDAL.EntryDetail.saveEntry(String strPath) in D:\ACHWINAPPLICATION\ACHDAL\EntryDetail.cs:line 164
   at ACHWINAPPLICATION.frmEntryDetails.btnSave_Click(Object sender, EventArgs e) in     D:\ACHWINAPPLICATION\ACHWINAPPLICATION\frmEntryDetails.cs:line 114
  at System.Windows.Forms.Control.OnClick(EventArgs e)
  at System.Windows.Forms.Button.OnClick(EventArgs e)
  at System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)
  at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
   at System.Windows.Forms.Control.WndProc(Message& m)
    at System.Windows.Forms.ButtonBase.WndProc(Message& m)
     at System.Windows.Forms.Button.WndProc(Message& m)
      at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
      at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
      at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg,       IntPtr wparam, IntPtr lparam)
      at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
 at                                      System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.      IMsoComponentManager.FPushMessageLoop(Int32 dwComponentID, Int32 reason, Int32 pvLoopData)
    at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
      at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
    at System.Windows.Forms.Application.RunDialog(Form form)
     at System.Windows.Forms.Form.ShowDialog(IWin32Window owner)
        at ACHWINAPPLICATION.frmMain.addEntry_Click(Object sender, EventArgs e) in            D:\ACHWINAPPLICATION\ACHWINAPPLICATION\frmMain.cs:line 553
         at System.Windows.Forms.ToolStripItem.RaiseEvent(Object key, EventArgs e)
          at System.Windows.Forms.ToolStripMenuItem.OnClick(EventArgs e)
         at System.Windows.Forms.ToolStripItem.HandleClick(EventArgs e)
         at System.Windows.Forms.ToolStripItem.HandleMouseUp(MouseEventArgs e)
           at System.Windows.Forms.ToolStripItem.FireEventInteractive(EventArgs e,     ToolStripItemEventType met)
       at System.Windows.Forms.ToolStripItem.FireEvent(EventArgs e,  ToolStripItemEventType met)
       at System.Windows.Forms.ToolStrip.OnMouseUp(MouseEventArgs mea)
         at System.Windows.Forms.ToolStripDropDown.OnMouseUp(MouseEventArgs mea)
         at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
        at System.Windows.Forms.Control.WndProc(Message& m)
      at System.Windows.Forms.ScrollableControl.WndProc(Message& m)
         at System.Windows.Forms.ToolStrip.WndProc(Message& m)
      at System.Windows.Forms.ToolStripDropDown.WndProc(Message& m)
        at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
           at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
      at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
  at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
 at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(Int32 dwComponentID, Int32 reason, Int32 pvLoopData)
    at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
     at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
      at System.Windows.Forms.Application.Run(Form mainForm)
     at ACHWINAPPLICATION.Program.Main() in D:\ACHWINAPPLICATION\ACHWINAPPLICATION\Program.cs:line 20
  at System.AppDomain._nExecuteAssembly(Assembly assembly, String[] args)
  at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
       at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
       at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
         at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadHelper.ThreadStart()