Error after selecting text in richtextbox

[C# Programming]

I am developing an application in C# and using richtextbox. After formatting the text when i select all the text it gives exception given below:

See the end of this message for details on invoking
just-in-time (JIT) debugging instead of this dialog box.

************** Exception Text **************
System.NullReferenceException: Object reference not set to an instance of an object.
at RichTextEditor.Editor.ucEditor.rtbDoc_SelectionChanged(Object sender, EventArgs e) in E:\MDCENTERWINAPP\Editor\ucEditor.cs:line 841
at System.Windows.Forms.RichTextBox.OnSelectionChanged(EventArgs e)
at System.Windows.Forms.RichTextBox.WmSelectionChange(SELCHANGE selChange)
at System.Windows.Forms.RichTextBox.WmReflectNotify(Message& m)
at System.Windows.Forms.RichTextBox.WndProc(Message& m)
at System.Windows.Forms.Control.ControlNative********OnMessage(Message& m)
at System.Windows.Forms.Control.ControlNative********WndProc(Message& m)
at System.Windows.Forms.Native********Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

Any Suggestion...............

Do good and have good.