null reference exception in richtextbox

Question

hai.

am writing dll for capture port pockets(like tcp sniffer) in vc++.in this application using thread for retrive pockets continuesly.and have event for send data to c# application.

in c# appliaction receive that pocket data and put richtextbox and store that data to .txt file from richtextbox every 10 seconds and clear textbox.

its running.but sometimes throw nullreference exception.

errors:

at System.Windows.Forms.RichTextBox.EditStreamProc(IntPtr dwCookie, IntPtr buf, Int32 cb, Int32& transferred)
   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.ControlNativeWindow.OnMessage(Message& m)
   at System.Windows.Forms.ControlNativeWindow.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)