AccessViolation happens in RichTextBox.ScrollToCaret

Exceptional exception tracking occurs when non-UI threads try to add their output to the RichTextBoxUI control in the main thread.

This exception occurs at random times, mainly when threads quickly call this method. This happens even in two threads without an interface.

Below is the code for the AppendLog method. It is in the form class of the main user interface. I create 2 threads and pass this method to them asAction<string> logDelegate

I even have syncobject.

  public void AppendLog(string message)
    {
        try
        {
            if (this.InvokeRequired)
            {
                this.Invoke(new Action<string>(this.AppendLog), message);
            }
            else
            {
                lock (_logSyncRoot)
                {

                    if (rtbLog.TextLength > 100000)
                        rtbLog.ResetText();

                    rtbLog.AppendText(message);
                    rtbLog.ScrollToCaret();
                }
            }
        }
        catch (Exception ex)
        {
            Logger.LogException(ex);
        }
    }

System.AccessViolationException: Attempted to read or write protected memory. This often indicates that another memory is corrupted.

  at System.Windows.Forms.UnsafeNativeMethods.CallWindowProc(IntPtr wndProc, IntPtr hWnd, Int32 msg, IntPtr wParam, IntPtr lParam)
  at System.Windows.Forms.NativeWindow.DefWndProc(Message& m)
 at System.Windows.Forms.Control.DefWndProc(Message& m)
at System.Windows.Forms.Control.WndProc(Message& m)
at System.Windows.Forms.TextBoxBase.WndProc(Message& m)
at System.Windows.Forms.RichTextBox.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.SendMessage(HandleRef hWnd, Int32 msg, Int32 wParam, Object& editOle)
at System.Windows.Forms.TextBoxBase.ScrollToCaret()
at MyApp.UI.OfflineAnalyzer.AppendLog(String message) in    D:\MyApp\Code\Charting\OfflineAnalyzer.cs:line 339