Exception occurred: Object reference not set to an instance of an object

System.NullReferenceException: Object reference not set to an instance of an object
in <0x00000> <unknown method>
in (wrapper managed-to-native) GLib.Object:gtksharp_object_newv (intptr,int,intptr[],GLib.Value[])
in <0x000c8> GLib.Object:CreateNativeObject (System.String[] names, GLib.Value[] vals)
in <0x00013> Gtk.Object:CreateNativeObject (System.String[] names, GLib.Value[] vals)
in <0x00010> Gtk.Widget:CreateNativeObject (System.String[] names, GLib.Value[] vals)
in <0x0005d> Gtk.FileChooserDialog:.ctor (System.String title, Gtk.Window parent, FileChooserAction action, System.Object[] button_data)
in <0x0008b> MonoDevelop.Gui.Widgets.FileSelector:.ctor (System.String title)
in <0x00039> MonoDevelop.Commands.OpenFileHandler:Run ()
in <0x0000b> MonoDevelop.Commands.CommandHandler:Run (System.Object dataItem)
in <0x00056> MonoDevelop.Commands.ActionCommand:DispatchCommand (System.Object dataItem)
in <0x00355> MonoDevelop.Commands.CommandManager:DispatchCommand (System.Object commandId, System.Object dataItem)