Error showing

System.NullReferenceException: Object reference not set to an instance of an object.
   at VidyalayaWin.RecordType3.BindDetailgridview() in f:\Project\VidyalayaWin\VidyalayaWin\Forms\ComputerPayBill\RecordType3.cs:line 821
   at VidyalayaWin.RecordType3.CBOmONTH_SelectedIndexChanged(Object sender, EventArgs e) in f:\Project\VidyalayaWin\VidyalayaWin\Forms\ComputerPayBill\RecordType3.cs:line 948
   at System.Windows.Forms.ComboBox.OnSelectedIndexChanged(EventArgs e)
   at System.Windows.Forms.ComboBox.WmReflectCommand(Message& m)
   at System.Windows.Forms.ComboBox.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.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
